Abstract: BitTorrent system suffers from free-riding because free-riding behavior has negative effect on the performance of the system. However, existing mechanisms used to counter free-riding in BitTorrent are not effective enough. In this paper, we propose a global-trust-based differentiated service scheme to deal with free-riding problem. In our scheme, tracker serves as a computation agent to calculate peers' global trust values. With these values, tracker identifies free-riders and contributive peers, and then disseminates the information of free-riders to free-riders and the information of contributive peers to contributive peers. Through such isolation, free-riders will have no chance to connect with contributive peers to obtain resources. Moreover, contributive peers are divided into high-contributors and normal-contributors. Tracker provides differentiated service for these peers according to their return ratios, which makes high-contributors achieve faster download bandwidth. Simulations show that the proposed scheme can significantly isolate and penalize free-riders, thus incentivizing peers to donate more upload bandwidth for the system.