This paper investigates power allocation strategies in an orthogonal frequency division multiplexing (OFDM) based decode-and-forward (DF) relay network consisting of one source, one relay and one destination. Different from conventional power allocation schemes which mainly focus on total throughput optimization, we aim at maximizing the energy efficiency with quality of service (QoS) guarantees. By simplifying a three-node relay network to a two-node relay network with equivalent channel gain, an effective energy efficient power allocation scheme with closed form solution based on water filling method is proposed to address the problem. Numerical results are shown to demonstrate the effectiveness of our proposed power allocation scheme.