The corrosion behavior of a 18Cr-ODS (oxide dispersion strengthened) steel exposed in supercritical water (600°C/25MPa) was investigated. The results indicate that the weight gain increases with the exposure time, following a parabolic law. A triple layer consisting of an outer layer, an inner layer and a diffusion layer forms on the sample. The phases of the outer layer are Fe 2 O 3 and Fe 3 O 4 . The inner layer and the diffusion layer contain (Fe, Cr) 3 O 4 and Cr 2 O 3 , respectively. Cracks, pores and exfoliations could be observed on the outer layer. Finally, the oxidation mechanism was discussed and the determining factors were given.