<SUBMISSION>
<ACCESSION-NUMBER>0001140361-08-020716
<TYPE>3
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20080829
<FILING-DATE>20080903
<DATE-OF-FILING-DATE-CHANGE>20080903
<ISSUER>
<COMPANY-DATA>
<CONFORMED-NAME>CALPINE CORP
<CIK>0000916457
<ASSIGNED-SIC>4911
<IRS-NUMBER>770212977
<STATE-OF-INCORPORATION>DE
<FISCAL-YEAR-END>1231
</COMPANY-DATA>
<BUSINESS-ADDRESS>
<STREET1>50 WEST SAN FERNANDO ST
<CITY>SAN JOSE
<STATE>CA
<ZIP>95113
<PHONE>4089955115
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>50 W SAN FERNANDO
<STREET2>SUITE 500
<CITY>SAN JOSE
<STATE>CA
<ZIP>95113
</MAIL-ADDRESS>
</ISSUER>
<REPORTING-OWNER>
<OWNER-DATA>
<CONFORMED-NAME>GRAVES KENNETH ANDREW
<CIK>0001444107
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>3
<ACT>34
<FILE-NUMBER>001-12079
<FILM-NUMBER>081054828
</FILING-VALUES>
<BUSINESS-ADDRESS>
<PHONE>713-830-2000
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>717 TEXAS AVENUE, SUITE 1000
<CITY>HOUSTON
<STATE>TX
<ZIP>77002
</MAIL-ADDRESS>
</REPORTING-OWNER>
<DOCUMENT>
<TYPE>3
<SEQUENCE>1
<FILENAME>doc1.xml
<DESCRIPTION>FORM 3
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0203</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2008-08-29</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000916457</issuerCik>
        <issuerName>CALPINE CORP</issuerName>
        <issuerTradingSymbol>CPN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001444107</rptOwnerCik>
            <rptOwnerName>GRAVES KENNETH ANDREW</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>CALPINE CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>717 TEXAS AVENUE, SUITE 1000</rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77002</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Interim Corp. Controller &amp; PAO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common stock, par value $0.001</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4000</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common stock, par value $0.001</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1100</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Option (Right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>16.90</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <value>2018-01-31</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>2500</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Option (Right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>18.38</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2018-03-05</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>4700</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Restricted stock awarded on February 6, 2008, pursuant to the Company's 2008 Equity Incentive Plan. The award vests and becomes nonforfeitable over a three-year period: 50% on February 6, 2009, 33% on February 6, 2010, and 17% on February 6, 2011.</footnote>
        <footnote id="F2">Restricted stock awarded on March 5, 2008, pursuant to the Company's 2008 Equity Incentive Plan. The award vests and becomes nonforfeitable over a three-year period: 33-1/3% on January 31, 2009, 33-1/3% on January 31, 2010, and 33-1/3% on January 31, 2011.</footnote>
        <footnote id="F3">Option granted on January 31, 2008, pursuant to the Company's 2008 Equity Incentive Plan. The option becomes exercisable over a three-year period: 50% on January 31, 2009, 33% on January 31, 2010, and 17% on January 31, 2011.</footnote>
        <footnote id="F4">Option granted on March 5, 2008, pursuant to the Company's 2008 Equity Incentive Plan. The option becomes exercisable over a three-year period: 33-1/3% on January 31, 2009, 33-1/3% on January 31, 2010, and 33-1/3% on January 31, 2011.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/  Kenneth Graves</signatureName>
        <signatureDate>2008-09-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
