Abstract: The oxidation behaviour of Incoloy800H alloy in 750℃ and 850℃ pure steam was investigated by discontinuous weighing method, X-ray diffraction, and scanning electron microscopy. The results show that the oxidation kinetics of the tested alloy followed a parabolic law; the oxidation accelerated rapidly with rising temperature. After oxidation in 750℃ steam, an oxide layer mainly composed of (Cr, Mn)₂O₃ was formed on the surface of the tested alloy, discontinuous bulgy Fe₃O₄ was distributed on the oxide layer and Cr in the matrix under the oxide layer was oxidized internally to Cr₂O₃. After oxidation in 850℃ steam, a duplex oxide layer, composed of a thin outer layer of (Fe, Mn)₃O₄ and a thick inner layer of (Cr, Mn)₂O₃, was formed on the surface of the tested alloy; beneath the oxide layer, Al was oxidized inernally to Al₂O₃ along grain boundaries in the matrix.