Abstract: The friction and wear of fine and coarse grain Al₂O₃ ceramics in sliding against a Si₃N₄ ball in room air were comparatively investigated. The results show that the 0.6 μm grain size Al₂O₃ ceramic/Si₃N₄ tribo-couple had better tribological performance (i.e. low friction, low wear rate and high critical load for wear transition) than that of 3.8 μm grain size Al₂O₃ ceramic/Si₃N₄ tribo-couple. SEM observations on the worn surfaces of the two Al₂O₃ ceramics revealed two distinct wear modes and evolution mechanisms. At a load of 15 N, the worn surface of 3.8 μm grain size Al₂O₃ ceramic underwent fracture and pullout of grain at the early stage, formation of the tribo-layer after a sliding duration of 100 s, an increasing area of the tribo-layer with prolonged time. The tribo-layer had characteristics, e.g. smooth, many interlinked cracks and mechanical mixing. No tribo-layer was found on the worn surface of 0.6 μm grain size Al₂O₃ ceramic at a load of 15 N. The evolution of the worn surface of 0.6 μm grain size Al₂O₃ ceramic was asperity level wear and cracks at the grain boundaries. Finally, the contribution of grain size effect and wear depth effect on the evolution of worn surface was discussed.