In this research, the effect of ball size distribution on the mill power draw, charge motion regime and breakage mechanism in a laboratory ball mill was studied using the discrete element method (DEM) simulation. The mill shell and crushing balls were made of Plexiglas® and compressed glass, respectively. Modeling was performed using Particle Flow Code 3D (PFC3D). Model parameters were back-calculated by comparing the power draws and images obtained from simulation and laboratory test works. After determining the model parameters, the mill was simulated in mill fillings of 15, 20, 25, 30, 35 and 40% with ball media of 2 and 2.5 cm in diameter. For every mill filling, the numbers of big and small balls were changed and 11 scenarios were chosen. The results showed that at a constant mill filling, the power draw was changed with changing the ball size distribution and for all mill fillings the maximum power draw occurred when the fraction of small balls was between 30-40%. The effect of ball size distribution increased with increasing mill filling and for the mill filling of 35%, the ball size distribution had the maximum effect on the power draw. When the mill charge contained mono-sized balls, the ball flow regime inside the mill transited to the cataracting and impact breakage was the main breakage mechanism. Increasing the fraction of big balls from 0 to 70% led the flow of balls into the cascading regime and breakage mechanism to attrition.