Abstract: Based on the results from the in-service inspection to the auxiliary piping welds of reactor coolant system in a nuclear power plant and the corresponding stress analysis reports in detail design phase, the typical flawed welds were determined to take fatigue analysis and assessments in this paper. According to the experimental results from WRC 502 and RCC-M code, the fatigue curve for considering the microfissure effects was present. Furthermore, the comparison of the transient cycle numbers recorded in the reactor operation in five years with that specified in detail design phase was taken, the fatigue analysis and assessment for the typical flawed weld were performed using the optimized fatigue method. The assessment results show that the fatigue failure of the auxiliary piping weld will not occur in ten years operation of nuclear power plant.