Owing to the superior properties of stainless steel it is pertinent to make use of it in various applications of automotive, aerospace, nuclear, chemical and cryogenic products. This paper describes a study of friction characteristics of SUS304 and SUS630 stainless steel disk specimens against silicon nitride Si3N4 ball in dry, grease-, and oil-lubricated conditions and wear characteristics in dry condition in sliding contact. The ultrasonic nanocrystal surface modification (UNSM) technology was applied to those disk specimens and its friction and wear properties were compared with polished one. The experiment was conducted with a pin-on-disk tribo tester using ball-on-disk contact geometry at room temperature. The experiment results show that in dry, greased- and oil-lubrication conditions the coefficient of friction of UNSM-treated specimens was lower and constant than polished specimens and in dry condition the wear rate was also smaller than polished one. Micro dimples made by UNSM treatment contribute those effects.