In file sharing P2P (peer-to-peer) networks, the service availability is seriously affected by peers’ voluntary actions. For example, there are many freeriders and malicious peers in P2P networks. However, the pure P2P networks don’t take the issue of freeriders and malicious peers as the inherent part of the topology design, and all the peers are symmetry in the topology. This paper proposes a reciprocal capacity based adaptive topology protocol for P2P networks, which takes account of the peer’s rational belief of maintaining connections. The simulation and analyses show that the resulting topology is incentive compatible to different types of peers. In addition, compared with the proposed similar scheme, it is also more efficient with less network cost.