Spatial diversity for wireless transmission requires more than one antenna at the transmitter. However, mobile devices are usually limited by size, so installation of multiple antennas increases the hardware complexity significantly. Due to the omnidirectional nature of wireless signal, a data transmission between a source node and a destination node can be overheard by many other neighbor nodes. By exploiting this characteristic, a number of recent research activities on cooperative Medium Access Control (MAC) have been devised where low data rate stations are assisted by the high data rate stations in forwarding data traffics. Therefore, wireless devices with a single antenna can effectively form a virtual array of antennas by sharing each other's' antennas in a multiuser environment. To facilitate cooperative communication in the data link layer requires a great deal of attention. In designing a cooperative MAC, selection of relay stations is the most important criterion. In this paper, we propose a distributed cooperative MAC protocol where a potential relay node initiates itself to participate in the cooperation by calculating supported data transmission rate between source to relay and relay to destination links. A mathematical analysis of our proposed scheme is derived and throughput of the proposed scheme is then compared with that of the existing IEEE 802.11 DCF MAC. Numerical results show that our proposed scheme can increase the throughput of any IEEE 802.11 low data rate station comprehensively.