A simulation of the three-dimensional motion of grinding media in the stirred media mill for the research of grinding mechanism has been carried out by 3-dimensional discrete element method (DEM). The movement of ball assemblies was graphically displayed with some snapshots from start of the milling to 0.20 s. From these simulation results, the grinding zone in the mill was confirmed to be distributed into two regions, which is near the stirrer and the side wall of mill around the stirrer. The power changing the rotation speed of stirrer was examined based on the micro interactive forces at all the contact points between ball-to-ball and between ball-to-stirrer. DEM is a very powerful tool for the microanalysis of movement of balls, which could not have been solved by a conventional experimental method.