Abstract: Study on three-dimensional ultrasonic assisted grinding force has been an empty field in ultrasonic machining area.The three-dimensional ultrasonic assisted grinding force is analyzed by using the macro-mechanics theory,and its mathematics model is established.The grinding force variations at different grinding parameters are evaluated.Verification experiment is conducted.It is found that under certain conditions,the three-dimensional ultrasonic assisted grinding force is almost 20% less than that of two-dimensional ultrasonic assisted grinding.The results also show that the grinding force decreases with the increase of wheel speed,and decreases significantly when wheel speed is more than 3000 r/min,while it increases with the increase of working feed speed and grinding depth,which has good compatibility with the theoretic analysis.