Abstract— Tree-based structure is widely used in peer-topeer streaming service and is the fundamental of many other structures, but it suffers a lot from the existence of free-riders. Based on the static analysis of the treebased structure, we further discuss the streaming service working in dynamic situation. We then present an incentive mechanism for tree-based live streaming service which requires the least cost to change the overlay by rotation, so it performs well in the dynamic situation. This incentive mechanism not only limits the damage of free-riders, but also provides better quality of service(QoS) for users with more contribution. In addition, we show this incentive mechanism can avoid cheating to some extent. We run a series of simulation experiments to show its validity.