Abstract: In this paper, the friction and wear tests of GCr15 steel-45# steel pairs were conducted on a "ball-on-disc" testing machine using stepwise loading method under drop-feed lubrication of 20# engine oil. The critical load showing the transition of friction and wear states were recorded under the condition of different sliding speed. Based on the test results a new P-V diagram was obtained. The structure of surface oxide film for different states was analysed by means of Scanning Auger Micro-probe (SAM). In order to determine accurately the type of oxide film at different depth, a new method by measuring the valence state M2,3 VV of Fe was employed. Thus, a complete structure model of oxide film was proposed and the mechanism of transition of friction states on P-V diagram can be explained appropriately by the continuous formation and breakdown of oxide film, which are related to many factors, such as, load, speed, loading method, hardness of substrate, surface roughness, lubrication mode, characteristics of lubricating oil etc.