In　OFDMA　systems　enhanced　with　user　cooperation,　the　proposed　cooperation　to　improve　the　frequency　efficiency　is　that　BS　transmits　uninterruptedly　and　the　relaying　users　decode-and-forward　the　first-slot-received　signal　to　the　objective　users　at　the　second　slot,　forming　the　two-slots　cooperative　transmit　with　spatial　diversity.　The　proposed　optimal　scheme　designs　the　utility　functions　based　on　the　service　character　and　the　sum　utility　maximization　is　realized　by　Lagrange　and　dual　decomposition,　which　brings　the　joint　optimizations　of　subcarrier-power　allocation　and　relay　selection.　The　simulation　results　show　that　the　user　requirement　can　be　guaranteed　and　the　high　utilization　is　better　achieved　when　the　proposed　scheme　is　used.