The particles suspended in fluid will move towards to nodes by acoustic radiation force in ultrasonic standing wave field. This theory can be applied to particles separation in engine oil. The forces on particles under ultrasonic wave field and the motion of particles were analyzed; the particle force motion analysis and particle's motion equation were established. The phenomenon of particles moving towards to wave nodes by the action of certain ultrasonic standing wave field was observed through experiment. The experiment result shows that the experimental results agreed well with the theoretical analysis. It is verified that the proposed method is feasible to be applied to separate the impurity from the viscous fluid such as engine oil.