In the seismic design of nuclear power plant, it is recently considered to apply the probabilistic method in addition to the deterministic method. The deterministic method secures the seismic safety of a nuclear power plant by veryfying that the calculated stress on the equipment is lower than allowable stress in the design criteria. The seismic probability safety assessment (Seismic PSA) method secures seismic safety by veryfying that the probability of reactor core failure is lower than a certain value. In seismic PSA, it is important to estimate the fragility of the equipment for precise assessment. Therefore, in this paper we dealt with a method using the Bayesian approach to generate a fragility curve of power plant from a multi-specimen test, and investigated the influence of the sample size driving the fragility evaluation test