Abstract: The oxidation mechanism and characteristics of steam-side oxide scales formed on the internal surface of reheater tubes made of T23 steel were investigated by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray diffraction (XRD). The results show that the oxide scale consisted of three layers: the inner layer was loose and porous heterosphere oxides containing Cr, V and W;the middle layer was the compact Fe3O4 with the columnar crystal microstructure;the outer layer was a relatively thin layer of Fe2O3. There was internal oxidation occurring in the interface between alloy matrix and inner oxide scale, and in the interface there were obvious bounds and small gaps, which made the oxides easily flake off during great temperature variation. The steam-side oxide characteristics, including the loose and porous inner layer and easy flaking off of whole oxides, decreased the anti-steam oxidation performance of T23 steel.