Wireless Sensor Networks (WSNs) have an excellent application to monitor environments such as military surveillance and forest fire. However, in many scenarios, WSNs are of interested to adversaries and these sensor nodes are deployed in open and unprotected environments, the security of WSNs is still an important issue. Traditionally cryptographic mechanisms such as authentication and encryption only address part of the problem of security in WSNs. In recent years, the use of reputation and trust systems has become an important mechanism for security in WSNs. In this paper, we develop a distributed agent-based trust model in WSNs. We also discuss some attacks in reputation and trust system, and offered some schemes to defense these attacks. The simulation results and analysis show that our model can detect the malicious nodes fast and accurately and efficiently prevent these attacks in WSNs.