Mobile Ad hoc Networks (MANET) is a new networking for wireless hosts. Because of self-organization, dynamic topology and openness of wireless communication, it makes them very attractive to attackers. Wormhole attack is one of the most severe threats to ad hoc networks. They create a higher level virtual tunnel between two malicious colluding nodes in the network. In this paper, we proposed a novel statistical analysis mechanism based on RTT (Round Trip Time) to detect and avoid wormhole attacks. It detected wormhole attacks during route setup procedure by statistically analyzing the route information and the transmission time between every two successive nodes along the established path before sending RREP to the source node. The wormhole will be recognize before it can do harm to the network in the route setup. And each node compute the RTT, it only introduces very limited overhead. Simulation results showed that the method can efficiently detect wormhole attacks and have better detection rate.