Corrosion behavior of silica glass was investigated in high-pressure water vapor and water at 300℃. A linear weight loss in water vapor and a parabolic weight loss in water were observed. The weight loss for the former was larger than that for the later. The fracture strength of the corroded silica glass increased with the content of molecular water in the glass. The strength reduction was observed for the corroded silica glass after dehydration at 400℃ for 1 day.