UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 10-Q

For the Quarterly Period Ended June 30, 2011

Commission File No. 0-21886

BARRETT BUSINESS SERVICES, INC.

(Exact name of registrant as specified in its charter)

(360) 828-0700

(Registrant’s telephone number, including area code)

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Website, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§ 232.405 of this chapter) during the preceding 12 months (or such shorter period that the registrant was required to submit and post such files).    Yes  x    No  ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer or a smaller reporting company (as defined in Rule 12b-2 of the Exchange Act).

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    Yes¨    No  x

Number of shares of common stock, $.01 par value, outstanding at July 29, 2011 was 10,129,399 shares.

BARRETT BUSINESS SERVICES, INC.

INDEX

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Part I – Financial Information

BARRETT BUSINESS SERVICES, INC.

Consolidated Balance Sheets

(Unaudited)

(In thousands, except per share amounts)

The accompanying notes are an integral part of these financial statements

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BARRETT BUSINESS SERVICES, INC.

Consolidated Statements of Operations

(Unaudited)

(In thousands, except per share amounts)

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BARRETT BUSINESS SERVICES, INC.

Consolidated Statements of Operations

(Unaudited)

(In thousands, except per share amounts)

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BARRETT BUSINESS SERVICES, INC.

Consolidated Statements of Cash Flows

(Unaudited)

(In thousands)

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 1 - Basis of Presentation of Interim Period Statements

The accompanying consolidated financial statements are unaudited and have been prepared by Barrett Business Services, Inc. (“Barrett”, “BBSI” or the “Company”), pursuant to the rules and regulations of the Securities and Exchange Commission. Certain information and note disclosures typically included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America have been condensed or omitted pursuant to such rules and regulations. In the opinion of management, the consolidated financial statements include all adjustments, consisting only of normal recurring adjustments, necessary for a fair statement of the results for the interim periods presented. The preparation of financial statements in conformity with generally accepted accounting principles (“GAAP”) requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results may differ from such estimates and assumptions. The consolidated financial statements should be read in conjunction with the audited financial statements and notes thereto included in the Company’s 2010 Annual Report on Form 10-K at pages F1 – F32. The results of operations for an interim period are not necessarily indicative of the results of operations for a full year.

Effective January 5, 2010, the Company formed a wholly owned insurance company, Ecole Insurance Company (“Ecole”). Ecole is a fully licensed insurance company holding a certificate of authority from the Arizona Department of Insurance. Ecole provides workers’ compensation coverage to the Company’s employees working in Arizona for claims occurring on or after March 1, 2010. The cost to capitalize the insurance company was approximately $6.1 million and is included in restricted marketable securities and workers’ compensation deposits in the consolidated balance sheet.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 1 - Basis of Presentation of Interim Period Statements (Continued)

Restatement of prior period

As described in Note 2 of the Consolidated Financial Statements in Item 15 in the Company’s 2010 Annual Report on Form 10-K, the Company restated its consolidated financial statements to correct an error related to legal expenses incurred for the administration of workers’ compensation claims by Associated Insurance Company for Excess (“AICE”), the Company’s captive insurance subsidiary formed January 1, 2007. The Company’s historical accounting method for legal expenses incurred for claims administration had been to recognize the expenses in the period incurred. The Company’s captive insurance subsidiary is subject to industry specific authoritative accounting guidance that requires the insurance subsidiary to include an estimate for such legal fees in its workers’ compensation claims liabilities. Therefore, the Company’s method of recording legal fees in the period incurred for claims administration was determined to be an error. As a result, the Company has presented its consolidated financial statements for the three and six months ended June 30, 2010, as restated to reflect the correction of this error. The Company also determined to make a conforming change to its accounting method for legal expenses incurred for the administration of workers’ compensation claims that arose prior to the formation of AICE. The impact of the correction of the error and change in accounting method for the three and six months ended June 30, 2010, which has been reflected throughout the consolidated financial statements and accompanying notes, is as follows:

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 1 - Basis of Presentation of Interim Period Statements (Continued)

Revenue recognition

We recognize revenue as services are rendered by our workforce. Staffing services are engaged by customers to meet short-term and long-term personnel needs. Professional employer organization (“PEO”) services are normally used by organizations to satisfy ongoing human resource management needs and typically involve contracts with a minimum term of one year, which cover all employees at a particular work site. Our PEO contracts are renewable on an annual basis and typically require 30 days’ written notice to cancel or terminate the contract by either party. Our PEO contracts provide for immediate termination upon any default of the client regardless of when notice is given. We report PEO revenues in accordance with the current accounting guidance for revenue recognition, which requires us to report such revenues on a net basis because we are not the primary obligor for the services provided by our PEO clients to their customers pursuant to our PEO contracts. Consequently, our PEO service fee revenues represent the gross margin generated from our PEO services after deducting the amounts invoiced to PEO customers for direct payroll expenses such as salaries, wages, health insurance and employee out-of-pocket expenses incurred incidental to employment and safety incentives. These amounts are also excluded from cost of revenues. PEO service fees also include amounts invoiced to our clients for employer payroll-related taxes and workers’ compensation coverage.

Marketable securities

As of June 30, 2011, the Company’s marketable securities consisted of tax-exempt municipal securities, corporate bonds and U.S. treasuries. The Company classifies municipal securities, U.S. treasuries, and certain of its corporate bonds as available for sale; they are reported at fair value with unrealized gains and losses, net of taxes, shown as a component of accumulated other comprehensive income (loss) in stockholders’ equity. In the event a loss is determined to be other-than-temporary, the loss will be recognized in the statement of operations. Certain of the Company’s corporate bonds are classified as held-to-maturity and are reported at amortized cost.

Allowance for doubtful accounts

The Company had an allowance for doubtful accounts of $463,000 and $374,000 at June 30, 2011 and December 31, 2010, respectively. The Company must make estimates of the collectibility of accounts receivable. Management analyzes historical bad debts, customer concentrations, customer creditworthiness, current economic conditions and changes in customers’ payment trends when evaluating the adequacy of the allowance for doubtful accounts. The Company deems an account balance uncollectible only after it has pursued all available assets of the customer and, where applicable, the assets of the personal guarantor.

Workers’ compensation claims

The Company is a self-insured employer with respect to workers’ compensation coverage for all of its employees (including employees subject to Professional Employer Organization (“PEO”) contracts) working in California, Oregon, Maryland, Delaware and Colorado. In the state of Washington, state law allows only the Company’s staffing services and internal management employees to be covered under the Company’s self-insured workers’ compensation program. Additionally, effective January 5, 2010, we formed a wholly-owned fully licensed insurance company in Arizona to provide workers’ compensation coverage to our employees in Arizona.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 1 - Basis of Presentation of Interim Period Statements (Continued)

Workers’ compensation claims (Continued)

To manage our financial exposure, in the event of catastrophic injuries or fatalities, we maintain excess workers’ compensation insurance through our wholly owned captive insurance company, Associated Insurance Company for Excess (“AICE”), with a per occurrence retention of $5.0 million, except in Maryland and Colorado, where our per occurrence retention is $1.0 million and $500,000, respectively. AICE maintains excess workers’ compensation insurance coverage with American Insurance Group, Inc. (“AIG”) between $5.0 million and $15.0 million per occurrence, except in Maryland, where coverage with AIG is between $1.0 million and $25.0 million per occurrence, and in Colorado, where the coverage with AIG is between $500,000 and statutory limits per occurrence. We continue to evaluate the financial capacity of our insurers to assess the recoverability of the related insurer receivables.

The Company has provided a total of $41.0 million and $39.3 million at June 30, 2011 and December 31, 2010, respectively, as an estimated future liability for unsettled workers’ compensation claims liabilities. Included in the foregoing liabilities are insured claims that will be paid by the Company’s former excess workers’ compensation insurer and for which the Company has reported a receivable from the insurer for the insured claims liability. Insured claims totaled $2.6 million at June 30, 2011 and $2.7 million at December 31, 2010, respectively. The estimated liability for unsettled workers’ compensation claims represents management’s best estimate based upon an actuarial valuation provided by a third party actuary. Included in the claims liabilities are case reserve estimates for reported losses, plus additional amounts based on projections for incurred but not reported claims and anticipated increases in case reserve estimates. Also included in these estimates are amounts for unallocated loss adjustment expenses, including legal costs. These estimates are continually reviewed and adjustments to liabilities are reflected in current operating results as they become known.

Safety incentives liability

Safety incentives represent cash incentives paid to certain PEO client companies for maintaining safe-work practices in order to minimize workplace injuries, thereby meeting agreed-upon loss objectives. The Company has provided $5.1 million at June 30, 2011 and $5.0 million at December 31, 2010 as an estimate of the liability for unpaid safety incentives. The incentive is based on a percentage of annual payroll and is paid annually to customers who meet predetermined workers’ compensation claims cost objectives. Safety incentive payments are made only after closure of all workers’ compensation claims incurred during the customer’s contract period. The liability is estimated and accrued each month based upon the incentive earned less the then-current amount of the customer’s estimated workers’ compensation claims reserves as established by the Company’s internal and third-party claims administrators, and the expected payout as determined by historical incentive payment trends. Safety incentive expense is netted against PEO services revenue in our consolidated statements of operations.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 1 - Basis of Presentation of Interim Period Statements (Continued)

Comprehensive income

Comprehensive income includes all changes in equity during a period except those that resulted from investments by or distributions to a company’s stockholders. Comprehensive income totaled $3.5 million and $2.1 million for the three months ended June 30, 2011 and 2010 and $9.0 million and $459,000 for the six months ended June 30, 2011 and 2010, respectively. Other comprehensive income (loss) refers to revenues, expenses, gains and losses that under GAAP are included in comprehensive income, but are excluded from net income as these amounts are recorded directly as an adjustment to stockholders’ equity. The Company’s other comprehensive income (loss) is comprised of unrealized holding gains and losses on its publicly traded marketable securities designated as “available-for-sale”, net of realized gains or losses included in net income.

Note 2 - Recent Accounting Pronouncements

In June 2011, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2011-05, “Comprehensive Income: Presentation of Comprehensive Income”. This new guidance requires the components of net income and other comprehensive income to be either presented in one continuous statement, referred to as the statement of comprehensive income, or in two separate, but consecutive statements. It also eliminates the current option to report other comprehensive income and its components in the statement of stockholders’ equity. While the new guidance changes the presentation of comprehensive income, there are no changes to the components that are recognized in net income or other comprehensive income under current accounting guidance. This new guidance is effective for the Company beginning January 1, 2012. As this guidance only alters the presentation of the components of comprehensive income, the adoption will not have an impact on the Company’s consolidated financial position or results of operations.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 3 - Basic and Diluted Earnings Per Share

Basic earnings per share are computed based on the weighted average number of common shares outstanding during the period. Diluted earnings per share reflect the potential effects of the exercise of outstanding stock options. Basic and diluted shares outstanding are summarized as follows:

Note 4 - Stock Incentive Plans and Stock-Based Compensation

The Company’s 2009 Stock Incentive Plan (the “2009 Plan”), which provides for stock-based awards to Company employees, non-employee directors and outside consultants or advisors, was approved by shareholders on May 13, 2009. No options have been issued to outside consultants or advisors. The number of shares of common stock reserved for issuance under the 2009 Plan is 1,000,000, of which the aggregate number of shares for which incentive stock options may be granted under the Plan is 900,000. No new grants of stock options may be made under the Company’s 2003 Stock Incentive Plan (the “2003 Plan”). At June 30, 2011, there were option awards covering 686,169 shares outstanding under the 2009 Plan, together with the 2003 Plan and its predecessor, the 1993 Stock Incentive Plan. Outstanding options under all three plans typically expire ten years after the date of grant and are generally exercisable in four equal annual installments beginning one year after the date of grant. During April 2011, 200,000 options were granted to officers and directors which become exercisable in eight equal annual installments beginning one year after the date of grant.

In accordance with accounting guidance for share-based payments, the Company recognizes compensation expense for options awarded under its stock incentive plans. Current accounting guidance requires the grant-date fair value of all share-based payment awards, including employee stock options, to be recognized as employee compensation expense over the requisite service period.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 4 - Stock Incentive Plans and Stock-Based Compensation (Continued)

The following table summarizes options activity in 2011:

The following table presents information on stock options outstanding for the periods shown:

The aggregate intrinsic value of stock options represents the difference between the Company’s closing stock price at the end of the period and the relevant exercise price multiplied by the number of options outstanding at the end of the period at each such price.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 4 - Stock Incentive Plans and Stock-Based Compensation (Continued)

The fair value of the stock-based awards, as determined under the Black-Scholes model, granted in the six months ended June 30, 2011 and 2010 were $2.1 million and $344,000, or $7.20 and $5.54 per share, respectively, estimated with the following weighted-average assumptions:

The following table summarizes stock-based compensation expense related to stock option awards for the three months and six months ended June 30, 2011 and 2010 (in thousands):

As of June 30, 2011 unrecognized compensation expense related to stock options was $2.5 million.

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 5 - Workers’ Compensation

The following table summarizes the aggregate workers’ compensation reserve activity (in thousands):

Note 6 - Fair Value Measurement

The Company has determined that its marketable securities should be presented at their fair value. Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date.

Factors used in determining the fair value of our financial assets and liabilities are summarized into three broad categories:

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 6 - Fair Value Measurement (Continued)

The inputs or methodology used for valuing securities are not necessarily an indication of the risk associated with investing in those securities.

The Company has determined that its U.S. treasuries component of its marketable securities falls into the Level 1 category. The Company has also determined that its municipal bonds and corporate bonds components fall into the Level 2 category. We classify our municipal bonds as level 2 due to our use of fair value pricing provided by an independent third-party based on observable market prices in less active markets or quoted market prices for securities with similar credit characteristics and rates traded in an active market. There were no assets or liabilities where Level 3 valuation techniques were used and there were no assets and liabilities measured at fair value on a non-recurring basis.

Marketable securities consist of the following investments (in thousands):

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BARRETT BUSINESS SERVICES, INC.

Notes to Consolidated Financial Statements (Unaudited) (Continued)

Note 6 - Fair Value Measurement (Continued)

The Company’s restricted marketable securities component of restricted marketable securities and workers’ compensation deposits consists of the following (in thousands):