Abstract: Air nozzle is widely used in mechanical manufacture engineering such as particle and water removal from a surface of a workpiece. It is interesting to know the condition that the best particle removal efficiency can be achieved. A particle image processing method using the relationship between the particle density and the reflected light intensity is proposed in this paper. The influencing factors on particle removal efficiency such as the air pressure, the impinging distance and the impinging angle are studied using the method. The best particle removal efficiency of nozzle under some operation conditions is obtained, which is coincident with the theoretical prediction.