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