Abstract: A nanostructured surface layer was fabricated on the surface of 45# steel by high pressure shot peening. Friction and wear tests of the nanocrystalline surface layer were implemented on a Model MMU-5G high temperature friction and wear tester. Microstructure of the surface layer was characterized by transmission electron microcopy (TEM). Differential scanning calorimetry (DSC) was employed to expatiate the thermal stability of the surface layer. Composition and morphology of the worn surfaces were also characterized by x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Because the high hardness of the surface layer by the strengthening of fine grains and working hardening dramatically reduced the real area of contact, the wear resistance of shot peening sample was better than that of as-received sample at temperatures ranging from room temperature to 200 oC. However, from 200 oC to 400 oC the as-received sample showed better tribological behavior that of the surface layer. It was because defects, e.g. grain boundary, dislocation, vacancy induced by the process of surface nanocrystallization were in favor of the diffusions of oxygen ions inwards and/or metal ions outwards, and hence accelerated the formation and breakdown of the loose oxide scale. Friction and wear of untreated and treated samples were comparable from 400 oC to 550 oC because of the recovery and growth of grain of the nanocrystalline layer.