Abstract: The microstructure of surface layer during fretting wear of 45, carbon steel in air at room temperature was studied. From the observation of fretting surface, cross section of fretted surface and wear debris by means of optical and scanning electron microscopy, it was found that the surface layer during fretting wear of carbon steel consists of three characteristic zones from outside to inside, the first zone was resulted from the oxidation of surface in fretting, the second was a serious plastic deformation layer, and a plastic deformation zone. The crystal grains near to surface have been smashed and its orientation was in the direction of fretting.The formation mechanism of flake-like oxide debris was discussed.