High-temperature steam oxidation causes accelerated oxidation damages for stainless steel tubes in boilers. A duplex oxide scale made of the outer scale of mainly Fe3O4, and the inner scale of mainly (Fe, Cr)(3)O-4, was formed on stainless steels in steam oxidation conditions. In this study, microstructure of the inner scale in the oxide scale formed by stream oxidation of 18Cr 8Ni stainless steel was analyzed by using the Auger electron spectroscopy. The inner scale consisted of three areas: Area I made of Cr2O3 on the inner scale/ alloy interface, Area II made of (Fe, Cr)(3)O-4 neighbor with Area I, and Area III consisted of (Fe, Cr)(3)O-4 matrix with dispersed Ni nano. particles. Continuity of Cr2O3 scale. Area I, was sometimes lost, resulting in the formation of the interface between (Fe, Cr)(3)O-4 and the stainless steel. Graphite is sometimes detected in only Area I, not in Area II and III, even after Ar sputtering for a long time. The existence of graphite is the evidence of permeation of CH4 gas into the scale/ alloy interface. Formation of fine Ni in Area III can be also explained with gas penetration through the cracks and post. healing of the outer scale.