We investigate a power allocation optimization problem under the scenario of multiple cooperative relays and one destination with multiple receiving antennas. We consider the relay protocol, "decode and forward" (DAF). An optimal power allocation scheme (OPA), through singular value decomposition (SVD) of the channel matrix, is developed among the relay nodes under a certain power constraint to achieve the maximum channel capacity. The expression of the optimal power allocation coefficient is derived using Lagrange multiplier. Simulation results show that the OPA can obtain higher capacity gain and lower outage probability comparing with the uniform power allocation (UPA) scheme. Moreover, we show that the OPA with DAF has better performance than it with AAF.