Abstract: In cooperative communications systems, allocating relay rationally is significant to promote the performance. However, the existing relay assignment methods only trend to maximize the capacity or the minimum transmission power, lack the comprehensive consideration of both capacity and transmission power so that the result of assignment is not the optimal. A relay allocation algorithm, called energy efficiency aware relay assignment algorithm (EERAA), is proposed with the objective of maximizing energy efficiency (ratio of capacity and transmission power). Firstly, EERAA algorithm optimizes the energy efficiency of collaboration link. Then, according to the result of maximum weight matching method. The base station allocates the optimal relay to source node, or source node communicating with base station directly. The simulation results verify that the energy efficiency of the proposed algorithm achieves a significant improvement compared with the existing methods.