Abstract: The effect of thermal aging at 400℃ for a longer time on the pitting corrosion of Z3CN20.09M cast austenite stainless steel (CASS) which is widely used in the primary coolant pipes of nuclear power plants was investigated by potentiodynamic anodic polarization and electrochemical impedance spectroscopy (EIS). The results show that the pitting potential and charge transfer resistance of the steel decrease with the increase of thermal aging time. It is interesting to note that the pitting potential and charge transfer resistance of specimens aged first and then annealed at 550℃ for 1 h increase up to those of the unaged. The results confirm that Cr-rich α' phase is dissolved in ferrite after annealing, but G phase still exists in ferrite at the same time, indicating that α' phase precipitated in ferrite is the main reason for the deterioration in pitting corrosion resistance of the thermally aged Z3CN20.09M steel.