Mutual exclusion is one of the well-studied fundamental primitives in distributed systems. However, the emerging P2P systems bring forward several challenges thatcan’t be completely solved by previous approaches. In this paper,we propose the Sigma protocol that is implemented inside a dynamic P2P DHT and circumvents those issues. The basic idea is to adopt queuing and cooperation between clients and replicas so as to enforce quorum consensus scheme. We demonstrate that this protocol is scalable with system size, robust to contention, and resilient to network latency variance and fault-tolerant.