Wireless sensor networks are composed of many small distributed sensor nodes, powered by small batteries which are typically never replaced, so it is essential to minimize the energy consumption in each node in order to increase network lifetime. one methode that significantly reduces the energy consumption in each node is cooperative communication. in this paper, we investigate power allocation to one source node and two partner nodes in an energy efficient cooperative communication. it is also shown that using two partners instead of one partner, can often increase the performance of the system, but it is not generally true.