The oxidation of ferritic steel and ferritic-martensitic steel was investigated by exposure to flowing and static supercritical water (SCW) at 550-600. °C. The oxidation kinetic curves follow parabolic and near-cubic rate equations for the samples exposed to flowing and static SCW, respectively. The phase analysis shows the presence of hematite, magnetite and spinel in flowing SCW while only the magnetite and spinel phases are identified in static SCW. The mechanism of the formation of hematite and the effect of the flow state of SCW on the time exponent of oxidation kinetics are discussed.