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Cooperative communication is a promising method to mitigate fading, achieve high spectrum efficiency and improve transmission capacity for wireless networks by means of spatial diversity. Recently increasing research is focused on cooperative medium access control (MAC) design to make full use of diversity gain. A relay-contention-free cooperative MAC (RCF-CMAC) protocol is proposed for wireless networks. In the protocol, a source node preselects two optimal candidate relays through its local relay information table, and sets different priorities to them in cooperative request-to-send (CRTS) packet according to relay efficiency which reflects the level of its saved time. Through overhearing the handshake between the source node and the destination node, the two candidate relays can acquire instantaneous transmission rate information among the source node, the destination node and themselves. Then they contend to become the final relay based on their priorities and associated rate information. The proposed protocol can rapidly select the optimal relay under current channel quality without contention collisions to cooperatively transmit data packets from all the potential relays, and consequently improve performance. Simulation results show that the proposed protocol outperforms IEEE 802.11 request-to-send/clear-to-send (RTS/CTS) and CoopMAC protocols.