The motion of balls in a two-dimensional vibration mill was investigated by means of a simulation method called Discrete Element Method, which made it possible to calculate the intensity and frequency of ball collisions. The effects of vibration conditions and fractional ball filling on the characteristics of ball collisions and the grinding rate were investigated by the simulation as well as the grinding experiment. Taking account of the strength of particles to be ground, effective breaking collision frequency was defined as the frequency of ball collisions greater than the particle breakage strength and compared with the grinding rate experimentally obtained. These results from the simulation and the grinding experiment agreed well regardless of the grinding conditions, Furthermore, the effects of the viscosity of liquid in the mill pot on the movement of balls, intensity and frequency of ball collisions were quantitatively discussed for a special case where the mill pot was completely filled with the liquid and balls.