Abstract: Design by analysis is one of the main methods for nuclear power equipment design. Key components are typically designed and operated using this safety-factor based on deterministic approach. The structure integrity is judged sufficiently providing safety margins inherent in the basic design. In practical engineering, nondeterministic variables exist, such as geometry, material properties, occurrence of beyond design basic loads etc. The reliability analysis was introduced into the design by analysis in this study, by considering the uncertainty of input variables involved in the evaluation. First, the limit state function of failure mode was established. Second, based on the theory of probability and statistics-based approach, the probability of failure or the reliability of nuclear key component was calculated. Finally, the sensitivity analysis was performed. The structural integrity of components is assessed by failure probability that has a positive meaning to the application of reliability theory in the design by analysis of ASME codes and standards.