<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0101</schemaVersion>

    <documentType>5/A</documentType><periodOfReport>2003-06-30</periodOfReport><dateOfOriginalSubmission>2003-06-30</dateOfOriginalSubmission>
    <form3HoldingsReported>0</form3HoldingsReported>

    <form4TransactionsReported>0</form4TransactionsReported>

    <issuer>
        <issuerCik>0000789019</issuerCik>
        <issuerName>MICROSOFT CORP</issuerName>
        <issuerTradingSymbol>MSFT</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId>
            <rptOwnerCik>0001054822</rptOwnerCik>
            <rptOwnerName>BURGUM DOUGLAS J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>Senior Vice President</officerTitle></reportingOwnerRelationship></reportingOwner><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>2002-07-31</value></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>8200</value><footnoteId id="F1"/></transactionShares><transactionValue><value>0</value></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>1641134</value><footnoteId id="F2"/><footnoteId id="F3"/><footnoteId id="F4"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeSecurity><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>1988-08-08</value><footnoteId id="F5"/></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>J</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F6"/></transactionCoding><transactionAmounts><transactionShares><value>0</value></transactionShares><transactionValue><value>0</value></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>55282</value><footnoteId id="F4"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>Joseph Ernest Burgum Trust</value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>1988-08-08</value><footnoteId id="F5"/></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>J</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F6"/></transactionCoding><transactionAmounts><transactionShares><value>0</value></transactionShares><transactionValue><value>0</value></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>55284</value><footnoteId id="F4"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>Jessamine Ada Burgum Trust</value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>1988-08-08</value><footnoteId id="F5"/></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>J</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F6"/></transactionCoding><transactionAmounts><transactionShares><value>0</value></transactionShares><transactionValue><value>0</value></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>55284</value><footnoteId id="F4"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>Thomas Alton Burgum Trust</value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>1988-08-08</value><footnoteId id="F5"/></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>J</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F6"/></transactionCoding><transactionAmounts><transactionShares><value>0</value></transactionShares><transactionValue><value>0</value></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>59632</value><footnoteId id="F4"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>By Spouse </value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>1988-08-08</value><footnoteId id="F5"/></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>J</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F6"/></transactionCoding><transactionAmounts><transactionShares><value>0</value></transactionShares><transactionValue><value>0</value></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>36214</value><footnoteId id="F4"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>By Son</value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>1988-08-08</value><footnoteId id="F5"/></transactionDate><transactionCoding><transactionFormType>5</transactionFormType><transactionCode>J</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F6"/></transactionCoding><transactionAmounts><transactionShares><value>0</value></transactionShares><transactionValue><value>0</value></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>5340</value><footnoteId id="F4"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>By 401(k)</value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><footnotes><footnote id="F1">Represents vesting stock award, which shall vest over 5 years at the rate of 20% on the first anniversary of the date of grant and then at the rate of 20% on each of the following four anniversaries thereafter, subject to continued employment.  Adjusted to reflect 2-for-1 stock split effective February 14, 2003.</footnote><footnote id="F2">This amendment filing is made to include 27 shares that were inadvertently missing in amount of securities beneficially owned as of June 30, 2003.</footnote><footnote id="F3">Includes 18 shares acquired on March 7, 2003 under the Microsoft dividend reinvestment plan and 930 shares acquired on June 30, 2003 under the Microsoft employee stock purchase plan.</footnote><footnote id="F4">This total represents shares owned as of June 30, 2003.</footnote><footnote id="F5">In the Section 16 Electronic Reporting Frequently Asked Questions released by the SEC on May 1, 2003, the SEC has designated &quot;08/08/1988&quot; as a &quot;dummy date&quot; until the electronic system is modified.  This line reports end of period indirect holdings for this reporting person, and 08/08/1988 is not a transaction date related to these securities.</footnote><footnote id="F6">There has been no transaction in the indirect holdings for this reporting person.</footnote></footnotes><ownerSignature><signatureName>Shauna L. Vernal, Attorney-in-Fact for Douglas J. Burgum</signatureName><signatureDate>2003-07-24</signatureDate></ownerSignature></ownershipDocument>

