For Internet based video broadcasting applications such as IPTV, the Peer-to-Peer (P2P) streaming scheme has been found to be an effective solution. An important issue in live broadcasting is to avoid playback buffer underflow. How to utilize the playback buffer and upload bandwidth of peers to minimize the freeze-ups in playback, is the problem we try to solve. In this work, we propose a successive water-filling (SWaF) algorithm for the video transmission scheduling in P2P live streaming system, to minimize the playback freeze-ups among peers. SWaF algorithm only needs each peer to optimally transmit (within its uploading bandwidth) part of its available video segments in the buffer to other peers requiring the content and pass small amount message to some other peers. Moreover, SWaF has low complexity and provable optimality. Numerical results demonstrated the effectiveness of the proposed algorithm.