Many P2P media streaming protocols assume that peers are all collaborative. They do not consider about the problem of selfish peers. We propose a score and monitoring based incentive mechanism for P2P media streaming system. In this mechanism each peer obtains contribution score in two ways: uploading data to others by itself and by its child peers. The peers with higher scores have better chances to download data. To maximize the score, peer can choose to monitor its child peers to stimulate sharing, accordingly improve the system performance. Through simulation, we verify that the proposed incentive mechanism can solve the long-term history score problem, increase the cooperative proportion and maintain higher system performance.