Based on Discrete Element Method(DEM), movements of steel balls in ball mill were simulated by Particle Flow Code in three Dimensions (PFC 3D), the motion states of all steel balls and balls with one diameter were discussed respectively at different ball filling rate. The results indicated that steel balls moved from cascading motion gradually to throwing motion with the increasing of rotary speed rate, and became centrifugal motion after rotary speed rate exceeded certain value. When rotary speed rate unchanged, the translation speeds from cascading to throwing or from throwing to centrifugal motion accelerated as steel balls’ diameter decreased. The segregation of steel balls was more obvious with the rising of rotary speed rate and lead to steel balls’ lamination that big balls moved in the inner and small balls moved in outer layer of the mill. Lamination of steel balls is benefical to crush different size minerals and improve grinding efficiency.