The mechanism of severe-mild wear transition and the role of wear particles in that process are studied using a pin-on-disk type wear test rig. Wear particles are continuously removed by air pressure. With the removal of wear particles, severe-mild wear transition does not occur. During the wear process, the worn surface of a pin specimen is sometimes observed with the optical microscope. It is observed that a mild wear surface is produced by back transfer of mild wear particles generated in the severe wear mode. onto the friction surface. When the entire area of apparent contact becomes a mild wear surface, the wear mode changes to mild. These results indicate that the mild wear mode is exhibited after the production of a mild wear surface; and that in severe-mild wear transition, Wear particles play the role of oxygen conveyer as an oxide needed for the generation of a mild wear surface.