Abstract: Opportunistic Distributed Space-Time Coding (O-DSTC) is a new type of cooperative communication. In the guarantee of the Symbol Error Rate (SER), the residual energy balanced cooperative partner selection algorithm of O-DSTC is presented based on the benefit function and regret function. When considering distance and only the statistics characteristics of channel information is available, the average SER of O-DSTC can not be computed, therefore the SER approaching formula is presented. The approaching formula selects the appropriate nodes as the cooperative partner candidates, and each candidate node sets the delay time distributedly. Meanwhile, each candidate node contends to become the cooperative partner. Simulation results demonstrate the reliability and effectiveness of the approaching formula. The simulation also shows that the cooperative partner selection algorithm can assure SER, enhance the minimum residual energy and reduce the contention access time.