This paper describes the correlation between particle reduction rate and charged amount of balls using alumina powder as a specimen. The reduction rate of particle size increased with increasing the rotation number. But in the case when ball was.charged more than half the amount of volume of the pot, there exist a critical rotation number where the smallest particle was produced. Beyond the critical rotation number where-balls moved along with inner wall of pot, the powder could not be ground effectively. The critical rotation number of pot depended on a charged amount of balls.