In order to solve “free rider” and “tragedy of the commons” problems in Peer-to-Peer(P2P) filesharing networks, this paper proposes a novel incentive mechanism based on debt theory and use gametheory to analyze its efficiency. While trying to maximize its own utility subjected to individualrationality, every peer allocates bandwidth resources efficiently according to its debt relationships withcompeting peers. The more contribution to the system, the better services the creditor will receive fromhis debtors. In contrast with most of the existing incentives, the whole distributed structure of thesystem and the practical locating algorithm avoid most of the complexities. Simulations show that theproposed mechanism increase the social utility of the whole P2P system significantly while isolatingmalicious peers from the network effectively.