Wireless Sensor Network (WSN) is developed with limited capabilities to operate at low cost, using low power, and has non-exist physical topology infrastructure. Somehow, WSN must be connectively reliable to transmit the events data between communicating nodes. In this paper, we propose a method to measure WSN reliability by using the Fault Tree Analysis to identify the specific desired event that contributes to WSN connectivity failure. The computation of Boolean algebra to derive the minimal cut set of network graph discovers the components that are prone to failure. The components need to be resolved to reduce the probability of failure in WSN connectivity. The proposed Fault Tree Analysis and its related Boolean algebra computation verifies the reliability of WSN connectivity is improved by the means of network redundancy design.