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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
 BTRy5ZN2hdZdPzuX48/fYGyMOxF9kf/d/4DRH7Kw2Ma1VIgMiIB4Tt2NPhrkCfVd
 cYNLjLHOY9tdCK4nU8mvbg==

<SEC-DOCUMENT>0001140361-05-005771.txt : 20050805
<SEC-HEADER>0001140361-05-005771.hdr.sgml : 20050805
<ACCEPTANCE-DATETIME>20050805155733
ACCESSION NUMBER:		0001140361-05-005771
CONFORMED SUBMISSION TYPE:	3
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20050803
FILED AS OF DATE:		20050805
DATE AS OF CHANGE:		20050805

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			CLEAN DIESEL TECHNOLOGIES INC
		CENTRAL INDEX KEY:			0000949428
		STANDARD INDUSTRIAL CLASSIFICATION:	INDUSTRIAL INORGANIC CHEMICALS [2810]
		IRS NUMBER:				061393453
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		300 ATLANTIC ST
		STREET 2:		STE 702
		CITY:			STAMFORD
		STATE:			CT
		ZIP:			06901
		BUSINESS PHONE:		2033277050

	MAIL ADDRESS:	
		STREET 1:		300 ATLANTIC ST
		STREET 2:		STE 702
		CITY:			STAMFORD
		STATE:			CT
		ZIP:			06901

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Copan Walter G
		CENTRAL INDEX KEY:			0001334198

	FILING VALUES:
		FORM TYPE:		3
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-27432
		FILM NUMBER:		051002830

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		203-327-7050

	MAIL ADDRESS:	
		STREET 1:		300 ATLANTIC STREET
		STREET 2:		SUITE 702
		CITY:			STAMFORD
		STATE:			CT
		ZIP:			06901
</SEC-HEADER>
<DOCUMENT>
<TYPE>3
<SEQUENCE>1
<FILENAME>doc1.xml
<DESCRIPTION>FORM 3
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2005-08-03</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000949428</issuerCik>
        <issuerName>CLEAN DIESEL TECHNOLOGIES INC</issuerName>
        <issuerTradingSymbol>CDTI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001334198</rptOwnerCik>
            <rptOwnerName>Copan Walter G</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>300 ATLANTIC STREET</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 702</rptOwnerStreet2>
            <rptOwnerCity>STAMFORD</rptOwnerCity>
            <rptOwnerState>CT</rptOwnerState>
            <rptOwnerZipCode>06901</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Vice President &amp; CTO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Option</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>1.40</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2006-07-25</value>
            </exerciseDate>
            <expirationDate>
                <value>2015-08-03</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>100000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Walter G. Copan</signatureName>
        <signatureDate>2005-08-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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