Using the vacuum mechanofusion apparatus, we coated Al₂O₃ powders with Cu powders. It was found that the longer the mechanofusion processing time. the smoother the surface of the composite particles. Such grain boundaries as typically seen in an anealed Cu bulk were observed in the smooth Cu covering layer when it was chemically etched. This implies that the temperature of the composite particles during the mechanofusion processing increases as high as Cu sinterring occurs. The surface temperature of each particle during the mechanofusion processing was estimated at about 1000 K from the result of sintering behavior of cold pressed Cu compacts.