With the wide application of wide area measurement system（WAMS）， its reliability becomes more and more important. Reliability is an important indicator to evaluate the performance of a WAMS. It means the probability for the system to complete data collection and data analysis. According to the structural characteristics of centralized and distributed systems， this paper makes an in-depth research on the WAMS reliability based on the reliability theory. A WAMS system can be divided into two sub-systems， namely monitor center system and data acquisition system. The availability of monitor center system can be evaluated by use of the Markov model， while the data acquisition system by use of series model， parallel model and k/n judgment model. The research model is validated with the reliability evaluation of IEEE 14 WAMS system.