Abstract: Based on the theory of fuzzy probability, this paper addresses the randomness of assessment parameters and fuzziness of failure modes of nuclear piping, and points that the failure probability of nuclear piping is a fuzzy failure probability actually. A method to compute the fuzzy failure probability of pressure piping is proposed. Compared with the conventional reliability estimation method, which neglects the existence of fuzzy failure areas, the method proposed in this paper provides a more complete assessment and could lead to a more accurate prediction of piping reliability.