The distinctive features of mobile ad hoc networks (MANETs), including dynamic topology and open wireless medium, may lead MANETs suffering from many security vulnerabilities. In this paper, using recent advances in uncertain reasoning, we propose a unified trust management scheme that enhances the security in MANETs. In the proposed trust management scheme, the trust model has two components: trust from direct observation and trust from indirect observation. With direct observation from an observer node, the trust value is derived using Bayesian inference. On the other hand, with indirect observation, also called secondhand information that is obtained from neighbor nodes of the observer node, the trust value is derived using the Dempster-Shafer theory. Combining these two components in the trust model, we can obtain more accurate trust values of the observed nodes in MANETs. And also merge encryption concept with the trust management system. Source send the encrypted message through the intermediate nodes. The destination will decrypt the message when it receives. We then evaluate our scheme under the scenario of MANET routing. Specifically throughput and packet delivery ratio can be improved significantly.