Experimental sintering studies on Al 2 O 3 powder (200nm and 600nm) were done at a heating rate of 1600°C/min. The microstructural changes of specimens were examined and corresponding detailed data on the densification and grain size as a function of sintering time were presented. The grain-growth transition behavior during sintering was discussed. The results showed that the neck growth caused principally by surface diffusion could be negligible within 2min. With subsequent increases of sintering time, grain growth promoted by grain boundary and lattice diffusion occurred.