Dry grinding of a gibbsite powder was conducted in air using a tumbling ball mill with mono-size of media (balls) ranging from 4.8 to 31.7 min diameter to investigate ball size effect on grinding rate. The impact energy of balls during grinding was calculated by the method proposed previously in order to correlate with the grinding rate. The grinding rate increases with an increase in the rotational speed of the mill, subsequently, it falls around the critical speed. The maximum grinding rate shifts toward higher rotational speed range as the ball size becomes large. In the range of rotational speed before the grinding rate falls, the grinding rate is improved when grinding with balls of 12.7 mm or less, while it is reduced when larger balls over 15.9 mm are used. A similar trend can be seen in the relation between the specific impact energy of balls and the rotational speed of the mill. The grinding rate is proportional to the specific impact energy of balls regardless of the ball size. Therefore, the specific impact energy of balls plays a significant role in governing the grinding rate in tumbling ball milling of the sample powder.