Abstract: Tribological behaviors of AISI 4340 steel have been investigated under dry sliding conditions at the room temperature of 25 ℃. The experiments were conducted on a CFT-Ⅰ multi-functional surface comprehensive performance tester. The morphology characteristics of the worn surface as well as the sub-surface were observed with a scanning electron microscope to analyze the friction and wear behavior. Induced elements transformation due to plastic deformation in sliding test has been detected by using an energy dispersive spectrometer. The results show that the main wear pattern appeared to be abrasive wear with mild adhesive wear and delamination. Various length, width and depth of worn tracks and the debris with different shape were detected. The worn surface showed mild oxidization and the wear debris was rich in carbon and mildly oxidized. Plastically deformed layer in the sub-surface was observed. And the degree of plastic deformation increased from subsurface to surface. Furthermore, grain boundary angles gradually converged to the worn surface and tended to be parallel to each other. The plastic deformation direction changed on account of the reciprocating movement, and converged at a localized zone in the middle of the worn surface.