Uncertain reasoning which is from artificial intelligence community, a unified trust management scheme that enhances the security in MANETs is proposed. In the proposed trust management scheme, the trust model has two components: trust from direct observation and trust from indirect observation. In direct observation from an observer node, the trust value is derived using Bayesian inference, when the full probability model can be defined. On the other hand, with indirect observation that is obtained from neighbor nodes of the observer node, the trust value is derived using the Dempster-Shafer theory (DST), which is another type of uncertain reasoning when the proposition of interest can be derived by an indirect method.