To study the effect of the particle size of industrial alumina powder on the properties of active alumina micropowder (α-Al2O3 micropowder), the fine powders with the median diameter of 50, 30, 10, and 5 μm, respectively, were obtained by grinding industrial alumina powder (the median diameter of 80 μm), and the active alumina micropowders were prepared by firing the industrial alumina powders before and after grinding at 1 320 ℃ for 5 h. The effect of the particle size of the industrial alumina powder on the microstructure and properties of the active alumina micropowder was researched. The results indicate that the initial particle size of the industrial alumina powder has a great influence on the grain morphology of the active alumina micropowder; when the median diameter is larger than 30 μm, the α-Al2O3 grains show worm like aggregates state after calcination and when it is around or less than 10 μm, the α-Al2O3 are round, spherical and well dispersed grains with the particle size of 0.3-1.0 μm. With the initial particle size of the industrial alumina powder decreasing, the α-Al2O3 phase transition rate increases and the true density gradually increases; moreover, the α-Al2O3 powder shows better dispersity and flowability.