Formal verification is mainly used to prove the correctness of safety-critical hardware and software systems. Localization problem in Wireless Sensor Network (WSN) is a hot research topic with many critical applications. Fuzzy Based Trilateration (FBT) algorithm is a simple and efficient localization technique that has been recently proposed. In this paper, we use PRISM model checker to formally verify the correctness of FBT algorithm. Our verification results show that the fuzzy rules used in the original FBT algorithm can be modified to reduce the average localization error without increasing the complexity of the algorithm. Our results also show that the localization error in the modified FBT (MFBT) algorithm is more likely to be acceptable than the original FBT algorithm.