Abstract: The deformation and microstructure evolution in running-in stage is of great importance for friction behavior of a tribo-system. In this experiment, the early running-in stage friction behavior of SUS304 stainless steel sliding against WC-Co ball were studied by dry sliding process with a load of 50 N. The roughness, hardness, oxide content, hardness, morphology, microstructure of the track surface in the first tens cycles were investigated. It is found that in the first 4 cycles of friction process, martensite phase transformation and severe deformation took place on the surface of SUS304 stainless steel, resulting in surface roughening and therefore wear debris generating. These microstructure evolutions had crucial influences on the friction behavior of the alloy in the following running-in stage. In the following friction process, the microstructure of surface and subsurface reached a relatively stable state gradually, resulting in the transformation of friction process from running-in stage to steady-state stage.