A grinding rate of a gibbsite powder sample was examined by using a planetary ball mill and it was well correlated kith the specific impact energy, which was calculated by Discrete Element Method (DEM). This implies a fact showing the confidence of the simulation result. Based on this result, an optimum rotation-to-revolution speed ratio and rotational direction of the pot to the revolution disk were investigated by the simulation work. A significant large specific impact energy is obtained when the mill pot is rotated at around the critical speed ratio in the counter direction to the disk revolution at around the critical speed ratio. (C) 2004 Elsevier B.V. All rights reserved.