Based on the kinematics analysis and the cutting depth calculation of single abrasive grit, the mathematics models of the cutting deforming force and the average cutting deforming force were made by introducing the unit grinding force, Fu. And the formula of the average cutting force during the grinding zone was calculated. The experiments on the grinding force ratio were conducted under conventional grinding(CG) mode and normal ultrasonic vibration assisted grinding(NUAG) mode. The results showed that the unit grinding force in NUAG was lower than that in CG under the same machining condition; the grinding force ration was decreased due to the normal ultrasonic vibration of the workpiece. And it was helpful to improve the machinablity of the hard-brittle material.