Wide area measurement system（WAMS） networks based on phasor measurement unit（PMU） are a combination of both PMUs and communication technologies. Consequently， they are likely to suffer many different types of failures. A model is described for assessing and comparing the reliability and availability of wide area networks. The model parameters include the reliabilities of the hardware and software components and the network topology considering node's failures. An evaluation method for WAMS network connectivity reliability is presented. All the minimal path sets of source-destination node pair in the network are calculated by using the minimal path set method. The connectivity reliability of source-destination node pairs is obtained through probabilistic analysis. By calculating a practical WAMS in Henan power grid， the availability of one PMU， and other reliability indices of WAMS are obtained， which can be used as a reference to evaluate the weakest component in the network comprehensively.