Abstract: The effect of thermal aging on properties of microzone mechanical and intergranular corrosion of stainless steel for nuclear power pipes were studied by means of thermal aging treatment, micro-mechanical properties testing, electrochemical testing and microstructure observation. The results show that with the prolongation of thermal aging time, the metallographic structure of the specimen dosen't change, the ferrite content and size dosen't change significantly, the ferrite phase hardness increases, while the austenite phase hardness basically dosen't change, and the intergranular corrosion resistance of the sample becomes worse. The hardness of ferrite in the thermally aged specimens for 10 000 h after annealed at 550℃ for 1 h returns to the level of the unaged specimens, and the resistance of intergranular corrosion improves. This is mainly due to the precipitation of Cr-rich α' phase which is formed by the spinodal decomposition in ferrite phase. The α' phase disappeares in the thermally aged specimens after annealed, and the microzone mechanical property is recovered. But, the speed of the disappearing of the Cr-depleted zone is slower than that of the dissolution of α'phase, thus the intergranular corrosion resistance is improved and not completely recovered.