The oxidation behavior of two Ferritic/Martensitic (F/M) steels including novel SIMP steel and commercial P91 steel were investigated by exposure to flowing deaerated supercritical water (SCW) at 700 C for up to 1000 h.The kinetic weight gain curves follow parabolic and near-cubic rate equations for SIMP and P91 steels,respectively.X-Ray Diffraction analysis showed the presence of magnetite and a spinel phase in flowing SCW for both steels.The morphology and structure of the oxide scales formed on these two steels were analyzed.The relationship between the microstructure and oxidation behavior and the reason that SIMP steel showed better oxidation resistance than P91 steel were discussed.