In this paper,we study the problem of optimal resource allocation for lifetime maximization in an orthogonal-frequencydivision multiplexing(OFDM)system with decode-and-forward relay.The goal is to minimize total energy cost of the system by jointly optimizing power allocation,subcarrier pairing and relay selection.We present a heuristic solution that is composed of two parts.The first part is an optimal power allocation approach to allocate power to a subcarrier pair of the source and the relay.The second part is a modified Hungarian algorithm to make subcarrier pairing and relay selection.Evaluations show that the presented scheme outperforms other schemes in the total transmitted data and the network lifetime. In this paper, we study the problem of optimal resource allocation for lifetime maximization in an orthogonal-frequencydivision multiplexing (OFDM) system with decode-and-forward relay. The goal is to minimize total energy cost of the system by jointly optimizing power allocation, subcarrier pairing and relay selection. We present a heuristic solution that is composed of two parts. first part is an optimal power allocation approach to allocate power to a subcarrier pair of the source and the relay. second part is a modified Hungarian algorithm to make subcarrier pairing and relay selection. Evaluation show that the presented scheme is outperforms other schemes in the total transmitted data and the network lifetime.