The corrosion behavior of ultra-pure austenitic stainless steel C25 after different heat treatment regimes, which used in spent fuel reprocessing equipment, was studied via immersion tests in 65% (mass fraction, hereafter the same) HNO₃ solution and 30% HNO₃ + 100 mg/L Cr⁶⁺ solution. The effect of sensitization temperature on corrosion behavior was analyzed. The results indicate that ultra-pure austenitic stainless steel C25 exhibited a very low uniform corrosion rate in 65% HNO₃, along with minimal intergranular corrosion tendency. In contrast, the uniform corrosion rate increased significantly in 30% HNO₃ + 100 mg/L Cr⁶⁺ solution, with more severe intergranular corrosion being observed. After sensitization treatment, immersion corrosion tests revealed that C25 stainless steel samples treated at 650 ℃ for 2 h (air-cooled) exhibited the highest intergranular corrosion susceptibility in the both solutions.