Abstract: In this study, degradation of fracture toughness due to thermal aging was considered in the probabilistic fracture mechanics analysis of nuclear pressure pipes. A predicting program for pipe break probability based on thermal aging embrittlement was developed. On the basis of experimental data of a pipe material, evolution of the fracture toughness at 280℃ and 330℃ were estimated using the Argonne National Laboratory（ANL） procedure. Failure probability of a pipe with a circumferential inner surface crack was calculated in two cases: consideration and without consideration of thermal aging.The result show that failure probability is higher when thermal aging embrittlement was considered. Also, cumulative failure probability at 280℃and 330℃ were compared. The results indicate that pipe will suffer more serious thermal aging and have higher failure probability when operating at a higher temperature.