<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2001-11-16</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000893813</issuerCik>
        <issuerName>PENN OCTANE CORP</issuerName>
        <issuerTradingSymbol>POCC</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001207639</rptOwnerCik>
            <rptOwnerName>MURPHY EMMETT M</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>201 MAIN STREET, SUITE 1555</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>FORT WORTH</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>76102</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1100000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Paradigm Capital / The Apogee Fund</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Purchase Warrants</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>3.66</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2001-11-16</value>
            </exerciseDate>
            <expirationDate>
                <value>2006-11-15</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>20000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Purchase Warrants</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>3.66</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2002-11-16</value>
            </exerciseDate>
            <expirationDate>
                <value>2006-11-15</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>10000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <remarks>Table I only:  The Reporting Person is the President of Paradigm Capital Corporation, a Texas Corporation, which in turn is the sole general partner of The Apogee Fund, L.P., a Delaware Limited Partnership.  All of the referenced securities are owned of record by The Apogee Fund, and beneficial ownership of such securities is attributable to the Reporting Person and Paradigm Capital solely by reason of their shared voting and disposition power with respect to assets of The Apogee Fund.  The Reporting Person disclaims beneficial ownership of the shares reported, except any shares attributable to him by virtue of his ownership interest in Paradigm Capital and Paradigm Capital's general partner interest in The Apogee Fund.</remarks>

    <ownerSignature>
        <signatureName>Emmett M. Murphy</signatureName>
        <signatureDate>2004-07-12</signatureDate>
    </ownerSignature>
</ownershipDocument>

