On the basis of a brief introduction about the recent progress of reliability assessment of complex systems, combined methods and state methods are introduced. In view of the shortcomings and deficiencies of these two kinds of methods, a reliability assessment method is put forward which based on fault tree analysis (FTA) and Bayesian network (BN). By applying the results of FTA of a complex system and adopting the transforming method of FTA to BN, BN model for reliability assessment of the complex system is established. A missile engine is taken for example and the effectiveness and the priority of the model in reliability assessment of system is verified.