In simulated pressurized water reactor environments, the corrosion test on 316 stainless steel was conducted for 40 to 4000 s, with 25 and 50 cc/kg-H2O dissolved hydrogen concentrations (NH and HH). It was found that the external Fe-rich oxide particles and the inner Cr-rich layer were simultaneously formed at the beginning of corrosion. However, the first formed Cr-rich layer was mainly Cr(OH)3 in the NH condition, while Cr2O3 in the HH condition. As exposure time increased, fraction of Cr2O3 increased due to transformation from Cr(OH)3 in both conditions. In addition, for HH condition, partial transformation to FeCr2O4 was observed.