The microstructural evolution during sintering of Al2O3 was investigated to determine the role of MgO additive, particularly when its concentration is very low (<200 ppm). Compared with specimens without MgO, a few Al2O3 grains were observed to grow enormously after the addition of 50 or 100 ppm MgO. When MgO content was increased to 200 ppm, on the other hand, the overall grain growth process was accelerated and many growing grains impinged on each other. In this case, therefore, a fine and unimodal grained microstructure was obtained. Sintering of Al2O3 in a MgO atmosphere further supported the promotion of grain growth by MgO. It is proposed that MgO promotes the grain growth of Al2O3 either by lowering the edge energy or by roughening the interface structure.