Abstract: Introducing relay to wireless networks results in severer energy consumption. The circuit power consumption of devices can not be neglected in energy efficient problem. In this paper, Orthogonal Frequency Division Multiplexing (OFDM)-based frequency-selective channel is considered in Decode-and-Forward (DF) relay link. An optimal allocation method is proposed, in which the transmit power in each hop is adaptively allocated to maximize the energy efficiency. The effects of the rate and power constraints on energy efficiency are also analyzed. Simulation results show that the proposed energy efficient power allocation method can achieve the highest energy efficiency while ensuring the high data rate.