To remove the brittle material in ductile state can help to improve the machined surface quality.Experiments of material removal mechanism for NdFeB are conducted by using tangential ultrasonic vibration assisted grinding,axial ultrasonic vibration assisted grinding and normal ultrasonic vibration assisted grinding.Based on the critical condition of brittle-ductile transition,the effects of different ultrasonic vibration modes on material removal mechanism are analyzed.Results indicate that the material is mainly removed in the form of plastic shear in axial ultrasonic vibration assisted grinding.The material is mainly removed in the form of transgranular fracture,plastic shear and little intergranular fracture in tangential ultrasonic vibration assisted grinding.While in normal ultrasonic vibration assisted grinding,the material is mainly removed in the form of fracture crash and the cracks remain in the machined surface.So axial ultrasonic vibration assisted grinding is the best machining method to realize the ductile grinding on brittle materials.