Peer clustering is a key operation for fully distributed systems such as wireless ad hoc networks and unstructured Peer-to-Peer (P2P) systems. The objective of peer clustering is to reconfigure the structure of an overlay network in such a way that the specific peers becomes closer without increasing the total number of links in the network. This paper proposes a peer clustering scheme for unstructured Peer-to-Peer (P2P) systems. The proposed scheme consists of an identification of critical links, local reconfiguration of incident links, and a retaliation rule. The simulation result indicates that the proposed scheme improves the performance of previous schemes and that a peer taking a cooperative action will receive a higher profit than selfish peers.