<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>4/A</documentType>
<periodOfReport>2003-12-04</periodOfReport>
<dateOfOriginalSubmission>2003-12-08</dateOfOriginalSubmission>
<notSubjectToSection16>0</notSubjectToSection16>
    <issuer>
        <issuerCik>0000856716</issuerCik>
        <issuerName>WESTERN GAS RESOURCES INC</issuerName>
        <issuerTradingSymbol>WGR</issuerTradingSymbol>
</issuer>
<reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001228362</rptOwnerCik>
            <rptOwnerName>STONEHOCKER WALTER L</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1099 18TH STREET, SUITE 1200</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>DENVER</rptOwnerCity>
            <rptOwnerState>CO</rptOwnerState>
            <rptOwnerZipCode>80202</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>1</isDirector>
<isOfficer>0</isOfficer>
<isTenPercentOwner>0</isTenPercentOwner>
<isOther>0</isOther>
<officerTitle></officerTitle>
<otherText></otherText>
</reportingOwnerRelationship>
</reportingOwner>
<nonDerivativeTable><nonDerivativeTransaction><securityTitle><value>Common Stock</value>
</securityTitle>
<transactionDate><value>2003-12-04</value>
</transactionDate>
<transactionCoding><transactionFormType>4</transactionFormType>
<transactionCode>S</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionAmounts><transactionShares><value>50000</value>
</transactionShares>
<transactionPricePerShare><value>45.82</value>
</transactionPricePerShare>
<transactionAcquiredDisposedCode><value>D</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<postTransactionAmounts><sharesOwnedFollowingTransaction><value>2409918</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature><directOrIndirectOwnership><value>I</value>
</directOrIndirectOwnership>
<natureOfOwnership><value>See footnote</value>
<footnoteId id="F1"></footnoteId>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeTransaction>
<nonDerivativeTransaction><securityTitle><value>Common Stock</value>
</securityTitle>
<transactionDate><value>2003-12-04</value>
</transactionDate>
<transactionCoding><transactionFormType>4</transactionFormType>
<transactionCode>S</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionAmounts><transactionShares><value>13000</value>
</transactionShares>
<transactionPricePerShare><value>45.85</value>
</transactionPricePerShare>
<transactionAcquiredDisposedCode><value>D</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<postTransactionAmounts><sharesOwnedFollowingTransaction><value>2396918</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature><directOrIndirectOwnership><value>I</value>
</directOrIndirectOwnership>
<natureOfOwnership><value>See footnote</value>
<footnoteId id="F1"></footnoteId>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeTransaction>
<nonDerivativeTransaction><securityTitle><value>Common Stock</value>
</securityTitle>
<transactionDate><value>2003-12-05</value>
</transactionDate>
<transactionCoding><transactionFormType>4</transactionFormType>
<transactionCode>C</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionAmounts><transactionShares><value>10142</value>
</transactionShares>
<transactionPricePerShare><value>39.75</value>
</transactionPricePerShare>
<transactionAcquiredDisposedCode><value>A</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<postTransactionAmounts><sharesOwnedFollowingTransaction><value>2407060</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature><directOrIndirectOwnership><value>I</value>
</directOrIndirectOwnership>
<natureOfOwnership><value>See footnote</value>
<footnoteId id="F2"></footnoteId>
</natureOfOwnership>
</ownershipNature>
</nonDerivativeTransaction>
</nonDerivativeTable>
<derivativeTable><derivativeTransaction><securityTitle><value>$2.625 Cumulative Convertible Preferred Stock</value>
</securityTitle>
<conversionOrExercisePrice><value>39.75</value>
</conversionOrExercisePrice>
<transactionDate><value>2003-12-05</value>
</transactionDate>
<transactionCoding><transactionFormType>4</transactionFormType>
<transactionCode>C</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionAmounts><transactionShares><value>8065</value>
</transactionShares>
<transactionPricePerShare><footnoteId id="F3"></footnoteId>
</transactionPricePerShare>
<transactionAcquiredDisposedCode><value>D</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<exerciseDate><footnoteId id="F3"></footnoteId>
</exerciseDate>
<expirationDate><footnoteId id="F3"></footnoteId>
</expirationDate>
<underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value>
</underlyingSecurityTitle>
<underlyingSecurityShares><value>10142</value>
</underlyingSecurityShares>
</underlyingSecurity>
<postTransactionAmounts><sharesOwnedFollowingTransaction><value>23735</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature><directOrIndirectOwnership><value>I</value>
</directOrIndirectOwnership>
<natureOfOwnership><value>See footnote</value>
<footnoteId id="F2"></footnoteId>
</natureOfOwnership>
</ownershipNature>
</derivativeTransaction>
</derivativeTable>
<footnotes><footnote id="F1">Reporting Person holds 1,262,575 shares of common stock of Western Gas Resources, Inc. directly and the remainder are held either by the Reporting Person and his spouse as joint tenants or by his spouse.</footnote>
<footnote id="F2">Reporting Person elected to convert 3,804 of the 15,000 shares of $2.625 Cumulative Preferred Stock ("Preferred Stock") held by him and his spouse elected to convert 3,804 shares of Preferred Stock held by her.  Reporting Person and his spouse also elected to convert 457 of the 1,800 shares of Preferred Stock jointly owned.  Following the transaction, Reporting Person owns 11,196 shares of Preferred Stock, the Reporting Person's spouse owns 11,196 shares of Preferred Stock, and the Reporting Person and his spouse jointly own 1,343 shares of Preferred Stock.</footnote>
<footnote id="F3">The Preferred Stock has no expiry date and is convertible at any time at the election of the holder.</footnote>
</footnotes>
<ownerSignature><signatureName>/s/ John C. Walter, as Attorney-in Fact for Walter L. Stonehocker</signatureName>
<signatureDate>2003-12-11</signatureDate>
</ownerSignature>
</ownershipDocument>

