This paper is to perform a probabilistic fracture mechanics assessment of piping welds in a nuclear power plant with considering residual stress effects.To assess the structural integrity objectively, the probabilistic fracture mechanics analysis code for nuclear piping is employed to estimate the leak probabilities of the welds.Considering the aging related degradation mechanism, crack growth rates in the welds subjected to fatigue and stress corrosion cracking are applied in Monte Carlo simulation.In order to verify the influence of overlay welding on stress improvement, a series of residual stress distributions in the welds are determined using the Finite Element Method.The geometries, material properties and operating loads of the piping system are based on those in the piping stress report.Present results show that the leak probabilities increase with operating loads are significantly related to welding residual stresses.Imposing compressive stresses at the inner surface of original pipe, weld overlay is an efficient way to avoid crack propagation during subsequent operation.In addition, the leak probability distributions can be regarded as the important factors for structural safety and operational stability evaluation of the piping system.