In a sliding contact between alumina ceramics in atmospheric environment without lubrication, the specific wear rate is significantly low at room temperature. To explore the mechanism of this mild wear of alumina, a series of ball-on-disk sliding wear tests were carried out at various temperatures. TEM observations of the wear surfaces show that a layer of very fine particles (10nm or less) is formed on the top surface in the mild wear regime, which is identified as a meta-stable phase γ-alumina. The layer appears to act as a lubrication film at the contact surface to reduce the wear loss. In a severe wear regime above 300°C, where the wear rate is relatively high, a surface structure consisting of a fine-particle layer and a deformed thin region beneath the layer is observed. Many microcracks exist in the surface structure, which may cause further material removal during a wear test, resulting in a high wear rate.