This work investigated the effect of grinding medium size on the wet milling performance in stirred mills using a combined numerical and experimental approach. Physical experiments were performed in a 1.4L stirred mill with the discs rotating at different speeds to grind aluminium hydroxide powders with different sized glass beads. The results showed that the grinding process followed the first order kinetics and the grinding rate increased with increasing disc speeds. While 4 mm glass beads had slightly larger grinding rates than 2 mm and 6 mm glass beads, the differences were generally within 5 , indicating their weak dependence. On the other hand, the energy consumption increased significantly for the large grinding media. Simulations based on the combined discrete element method (DEM) and computational fluid dynamics (CFD) approach were performed under similar conditions. The simulated flow patterns and power draws were comparable to those measured. The total impact energy, which is the summation of all collision energy, had a unified power law relationship with the grinding rate for all grinding conditions. It is expected that the ratio of total impact energy and input power, which decreased for larger grinding media in the present study, can be used to describe the grinding efficiency.