A method for simulating the motion of balls in tumbling ball mill under wet condition is investigated. The simulation method is based on the three-dimensional discrete element method (DEM) and takes into account the effects of the presence of suspension, i.e., drag force and buoyancy. The impact energy on balls' collision, which enables us to provide useful information for predicting the grinding rate, was calculated from the simulation work. Actual ball motion in the suspension in the mill was observed through a transparent mill lid made of acrylic acid resin at different solid concentrations and the suspension was made of water with powder samples such as gibbsite, limestone and talc. Grinding tests of the gibbsite sample using two laboratory tumbling ball mills made of stainless steel and using alumina with balls made of steel, alumina and glass were conducted and the grinding rate was determined by measuring the 50% particle size of the product. The specific impact energy calculated from the simulation work correlates well with the grinding rate of the sample, irrespective of the grinding conditions, so that the energy is the key factor controlling the grinding phenomena in wet milling. (C) 2004 Published by Elsevier B.V.