Abstract: A new method based on Bayesian network was proposed to assess the mechanical system reliability because of the limitation of fault tree used in reliability assessment. A fault tree was built first according to the block diagram of system reliability, then it was mapped onto a Bayesian network to generate a conditional probability distribution table so as to assess the system reliability by inference of Bayesian network. The method is able not only to compute the reliability indices of mechanical system but also to analyze easily the effects of a certain components on the system reliability to recognize the vulnerable positions of the system. The validity of the method proposed is verified by exemplification.