This paper presents HyMoNet, a peer-to-peer hybrid multicast overlay network for efficient live media streaming. The system comprises two parts: service infrastructure and user space. Service infrastructure contains the data stream servers manually deployed in the backbone network by the service provider. In user space, all users are dynamically classified into several tree structures, in which data is dispatched successively. With the user space management algorithm, the system can detect IP multicast service intelligently and allow users to join and to leave dynamically. NAT is an obstacle for widespread of p2p system, so a NAT pass strategy is designed to overcome this problem. The system guarantees the reliable data transmission which is friendly to TCP data flow. HyMoNet is now deployed in Internet providing real-time TV program streaming for the public.