Numerical analysis has been carried out to investigate particles behavior characteristics during the grinding process in the air. Grinding particles behavior is largely affected by many parameters such as operating conditions and process room geometry. Transient particle motion in the air is predicted, and the effects of particle diameter and device rotational speed on the behavior characteristics were compared. When particle size is not large enough, they shortly moves in a tangential direction of the device rotation, and then are mostly influenced by the air flow in the room. These results could be applicable to the optimal design of the grinding process room.