Sliding wear mechanisms of the solution treated 18Cr-18Mn-2Mo-0.9N high nitrogen steel (HNS) were studied. Room-temperature dry sliding wear tests of the steel were carried out at various applied loads to explore the mechanism as a function of the load. The wear rate of the steel increased with the increase of the load; however, the increase rate was not constant. The rate increased slowly at low loads, rapidly at intermediate loads, and finally, the increase-rate became low again at high loads. Worn surfaces, their cross sections, and wear debris were examined, and phases of the heat-treated HNS as well as the wear debris were identified to find out the mechanism. The wear of the steel was found to be controlled by the tribo-oxidation, strain-induced phase transformation, and reverse transformation due to temperature rise on a wearing surface. The influence of each mechanism on the rate varied depending on the magnitude of the applied load.