Abstract: Microhardness on the surface and sub-surface, the martensitic transformation on the wear trace were investigated after the friction tests of SUS 304 austenitic stainless steel at different conditions using Ball-on-Disc test method. X-ray diffraction, optical micrograph and microhardness measurement techniques were employed in the present paper. The microhardness of the materials under the wear traces increased with pressure but it decreased when the distance to the surface increased. The friction-induced transformation of austenite to martensite on the worn surface was observed, and the amount of friction-induced martensite decreased when the pressure and the slid- ing speed increased. The increasement of hardness in the friction layer was due to the integrating actions of fined grain, transformed martensite and a high density of dislocations.