An adaptive relay selection algorithm(ARS) is proposed to optimize the performance of decode-and-forward cooperative communication networks, both system outage probability and average numbers of cooperative relays are analyzed. When the direct transmission between source and destination is fail, according to the channel quality, the destination orders relay nodes in the decode set, and the ordered relays are included in cooperation in turn until the instantaneous received SNR at destination is not smaller than the preset SNR threshold. Simulation results show that, the proposed algorithm can get optimal outage performance, decrease the average numbers of cooperative relays efficiently, to incrense bandwidth efficiency and reduce system overhead.