-----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>0000775345-08-000029.txt : 20080409
<SEC-HEADER>0000775345-08-000029.hdr.sgml : 20080409
<ACCEPTANCE-DATETIME>20080409162257
ACCESSION NUMBER:		0000775345-08-000029
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20080409
FILED AS OF DATE:		20080409
DATE AS OF CHANGE:		20080409

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			UNITED BANCORP INC /MI/
		CENTRAL INDEX KEY:			0000775345
		STANDARD INDUSTRIAL CLASSIFICATION:	STATE COMMERCIAL BANKS [6022]
		IRS NUMBER:				382606280
		STATE OF INCORPORATION:			MI
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		205 E CHICAGO BLVD
		STREET 2:		PO BOX 248
		CITY:			TECUMSEH
		STATE:			MI
		ZIP:			49286
		BUSINESS PHONE:		5174238373

	MAIL ADDRESS:	
		STREET 1:		205 E CHICAGO BLVD
		STREET 2:		P O BOX 248
		CITY:			TECUMSEH
		STATE:			MI
		ZIP:			49286

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			BOYSE STEPHANIE H
		CENTRAL INDEX KEY:			0001425418

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-16640
		FILM NUMBER:		08747919

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		517-423-1760

	MAIL ADDRESS:	
		STREET 1:		P O BOX 248
		CITY:			TECUMSEH
		STATE:			MI
		ZIP:			49286
</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>2008-04-09</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000775345</issuerCik>
        <issuerName>UNITED BANCORP INC /MI/</issuerName>
        <issuerTradingSymbol>UBMI.OB</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001425418</rptOwnerCik>
            <rptOwnerName>BOYSE STEPHANIE H</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>P O BOX 248</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>TECUMSEH</rptOwnerCity>
            <rptOwnerState>MI</rptOwnerState>
            <rptOwnerZipCode>49286</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock $0.00 Par</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3270</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Deferred Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2008-04-09</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>123.6842</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>19</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>123.6842</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1866.383</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The reporting person has received an exempt award of deferred stock units under United Bancorp's Director Retainer Stock Plan.  Under the Plan, a director may elect to defer payment of all or a portion of his future cash retainer for serving on the board of directors of United Bancorp, Inc., United Bank &amp; Trust or United Bank &amp; Trust - Washtenaw, but not for serving on any committee of the board, by electing to receive in lieu of such cash retainer deferred stock units.  The amount of deferred stock units is equal to the amount of the cash retainer that was elected to be deferred, divided by the market price of the common stock of United Bancorp common stock on the date such cash retainer would otherwise have been paid.  The deferred stock units are awarded on the date such cash retainer would otherwise have been paid (quarterly in arrears) and are immediately vested.  Each deferred stock unit is the economic equivalent of one share of common stock.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Randal J. Rabe</signatureName>
        <signatureDate>2008-04-09</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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