<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>3</documentType>
<periodOfReport>2003-09-11</periodOfReport>
    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001029023</issuerCik>
        <issuerName>SYNTROLEUM CORP</issuerName>
        <issuerTradingSymbol>SYNM</issuerTradingSymbol>
</issuer>
<reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001263208</rptOwnerCik>
            <rptOwnerName>EDMONSON RICHARD L</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1350 S. BOULDER</rptOwnerStreet1>
            <rptOwnerStreet2>STE. 1100</rptOwnerStreet2>
            <rptOwnerCity>TULSA</rptOwnerCity>
            <rptOwnerState>OK</rptOwnerState>
            <rptOwnerZipCode>74119</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector>
<isOfficer>1</isOfficer>
<isTenPercentOwner>0</isTenPercentOwner>
<isOther>0</isOther>
<officerTitle>VP, Secretary, General Counsel</officerTitle>
<otherText></otherText>
</reportingOwnerRelationship>
</reportingOwner>
<derivativeTable>
<derivativeHolding>
<securityTitle>
<value>Incentive Stock Option (right to buy)</value>
</securityTitle>
<conversionOrExercisePrice>
<value>2.50</value>
</conversionOrExercisePrice>
<exerciseDate>
<value>2004-08-14</value>
<footnoteId id="F1"/>
</exerciseDate>
<expirationDate>
<value>2013-08-14</value>
</expirationDate>
<underlyingSecurity>
<underlyingSecurityTitle>
<value>Common Stock</value>
</underlyingSecurityTitle>
<underlyingSecurityShares>
<value>100000</value>
</underlyingSecurityShares>
</underlyingSecurity>
<ownershipNature>
<directOrIndirectOwnership>
<value>D</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value></value>
</natureOfOwnership>
</ownershipNature>
</derivativeHolding>
</derivativeTable>
<footnotes>
<footnote id="F1">The stock option becomes exercisable in three equal annual installments, commencing one year after the Transaction Date (date of grant).</footnote>
</footnotes>
<remarks></remarks>
<ownerSignature>
<signatureName>Carla S. Covey for Richard L. Edmonson</signatureName>
<signatureDate>2003-09-11</signatureDate>
</ownerSignature>
</ownershipDocument>

