-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
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<SEC-DOCUMENT>0000716039-05-000088.txt : 20050705
<SEC-HEADER>0000716039-05-000088.hdr.sgml : 20050704
<ACCEPTANCE-DATETIME>20050705183714
ACCESSION NUMBER:		0000716039-05-000088
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20050630
FILED AS OF DATE:		20050705
DATE AS OF CHANGE:		20050705

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			ARNOLD CRAIG
		CENTRAL INDEX KEY:			0001278074

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-08483
		FILM NUMBER:		05938532

	MAIL ADDRESS:	
		STREET 1:		1111 SUPERIOR AVE
		CITY:			CLEVELAND
		STATE:			OH
		ZIP:			44114

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			UNOCAL CORP
		CENTRAL INDEX KEY:			0000716039
		STANDARD INDUSTRIAL CLASSIFICATION:	CRUDE PETROLEUM & NATURAL GAS [1311]
		IRS NUMBER:				953825062
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			0901

	BUSINESS ADDRESS:	
		STREET 1:		2141 ROSECRANS AVE
		STREET 2:		STE 4000
		CITY:			EL SEGUNDO
		STATE:			CA
		ZIP:			90245
		BUSINESS PHONE:		3107267600

	MAIL ADDRESS:	
		STREET 1:		2141 ROSECRANS AVE
		STREET 2:		STE 4000
		CITY:			EL SEGUNDO
		STATE:			CA
		ZIP:			90245
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>primary_doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-06-30</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000716039</issuerCik>
        <issuerName>UNOCAL CORP</issuerName>
        <issuerTradingSymbol>ucl</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001278074</rptOwnerCik>
            <rptOwnerName>ARNOLD CRAIG</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1111 SUPERIOR AVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CLEVELAND</rptOwnerCity>
            <rptOwnerState>OH</rptOwnerState>
            <rptOwnerZipCode>44114</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock / $1.00 Par Value</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <transactionDate>
                <value>2005-06-30</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>433.0626</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>58.1902</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>7892.5071</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock / $1.00 Par Value</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <transactionDate>
                <value>2005-06-30</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>17.309</value>
                    <footnoteId id="F3"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>58.1902</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>7909.8161</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Common Stock has attached Preferred Stock Purchase Rights.  If and when the Rights become exercisable,the holders would initially be entitled to purchase one one-hundredth of a share of Series B Junior Participating Preferred stock at a purchase price determined pursuant to the Rights Agreement.</footnote>
        <footnote id="F2">Stock Units credited for voluntary deferral of director fees under the 2004 Directors' Deferred Compensation and Restricted Stock Unit Award Plan.</footnote>
        <footnote id="F3">Dividend equivalent credited as Stock Units under the 2004 Directors' Deferred Compensation and Restricted Stock Unit Award Plan</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Paul R. Moore, Asst Sec'y, Attorney-In-Fact</signatureName>
        <signatureDate>2005-07-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----