We introduce a novel power allocation scheme for decode-and-forward relaying system with partial channel state (CSI) information, i.e., the source knows full CSI of source-relay link but only statistical CSI of source-destination and relay-destination links. Our objective is to minimize the outage probability by jointly allocating the transmit power between the source and relays. To avoid exhaustive search, the MAOP scheme and the MMS scheme are proposed to approach the optimal allocation in the high and low signal-to-noise ratio regimes, respectively.