Mobile ad hoc networking (MANET) has become an exciting and important technology in recent years. In this paper, we propose a distributed intrusion detection approach based on timed automata. We construct the timed automata by the way of manually abstracting the correct behaviours of the node according to the routing protocol. The monitor nodes can verify every node's behaviour by timed automata, and validly detect real-time attacks without signatures of intrusion or trained data.