An ad hoc network is a kind of wireless network without centralized administration or fixed network infrastructure, in which nodes perform routing discovery and routing maintenance in a self-organized way. However, this flexible network topology introduces a lot of security problems and the existing routing protocols for ad hoc networks such as AODV has no effective measures to prevent themselves from being attacked. Some researchers have proposed several secure routing protocols to protect the ad hoc networks. But most of these secure routing protocols need some centralized units or some trusted third parties to issue digital certificates or monitor network traffics, which actually destroy the self-organization nature of ad hoc networks. In this paper, we propose a secure routing protocol based on our trust mechanism. Each node in this ad hoc network has its opinions about some other nodes' trustworthiness, which are obtained by directly communicating with other nodes or by combining other nodes' recommendations. Then the node will decide whether to exchange routing information with another according to its opinion about that nodes' trustworthiness.