In the present work, high temperature oxidation of HP40 alloy was carried out at 1050°C under H 2 -H 2 O and air atmospheres; the influence of atmosphere on surface morphology and composition was studied. Octahedral crystals with considerable spalled regions are present on the surface of alloy oxidized under air, the oxide scale composes of MnCr 2 O 4 , Cr 2 O 3 and (Fe, Ni)Cr 2 O 4 and spalled regions exhibit base alloy and SiO 2 -rich regions. The surface of alloy oxidized under H 2 -H 2 O is fully covered by small granular crystals and blade-type structures without spallation, and the oxide scale composes of MnCr 2 O 4 and Cr 2 O 3 . Moreover, X-ray photoelectron spectroscopy analysis shows considerable difference in chemical valence states of Mn, Cr and O elements on both alloy surfaces, and hydroxyl compounds exist on the alloy oxidized under H 2 -H 2 O atmosphere.