Quality of service provisioning as well as limited energy of nodes, are two challenging issues in the performance enhancement of wireless communication networks. In this way, cooperative communication networks with RF energy harvesting capability which are interesting to researchers are proposed respectively to address such issues. The proposed protocols in such networks are mainly based on power splitting (PS) and time switching (TS) protocols that are usually suitable for the networks with no source-destination direct link. It should be noted that the relay traffic impacts on the relay as well as network performance. Most studies in such networks focus on the throughput maximization whereas other performance metrics such as network lifetime and packet delay have received less attention. To the best of our knowledge, the effect of both source-relay direct link and the relay traffic on lifetime and packet delays are not addressed simultaneously. Considering such issues, in this thesis a new policy is proposed for the relay in a cooperative communication network with RF energy harvesting capability. The proposed policy is evaluated in a three-node cooperative network under three scenarios in terms of source node energy constraint. It is assumed that in the first scenario the source node has unlimited energy. However, in both second and third scenarios the source node has limited energy where in the third one the source node has also the capability of energy harvesting from the relay signal. First, by modeling of the system based on Markov chains, data transmission probability of both source and relay nodes are calculated. Next the network effective lifetime is introduced and its average value is derived from a new analytical method based on discrete phase-type distribution model and also a simple proposed analytical method. The simulation results show good accuracy of both proposed analytical methods. In the following to find the optimum relay policy, the effect of energy harvesting probability on the average network effective lifetime as well as packet delay of source and relay nodes is investigated. The effect of other parameters such as the nodes initial energy and data buffer capacity on the network performance is evaluated too. Finally, it is shown that the proposed policy outperforms TS and PS protocols in the third scenario. Key words_ RF Energy harvesting , energy harvesting probability, Markov chain, discrete phase-type distribution model, cooperative communication.