This paper discusses the feasibility of improving machining efficiency of sapphire substrate by using two-dimensional (2D) ultrasonic vibration assisted grinding. An elliptic ultrasonic vibrator is designed and produced by bonding a piezoelectric ceramic device (PZT) on a metal elastic body (stainless steel, SUS304). The sapphire substrate is fixed onto the top face of the vibrator and ultrasonically vibrates in 2D vibration mode when the PZT is excited by two alternating current voltages with a phase difference. A grinding apparatus mainly composed of the ultrasonic vibrator is constructed, and experiments are performed with lateral modulation of elliptic ultrasonic vibration vertical to the grinding direction. Both the grinding forces and the ground work surface are measured and examined. Experimental results show that the grinding force decreases significantly and the resulted surface is improved in certain degree with the ultrasonic vibration compared to those of conventional grinding without ultrasonication. This indicates that the high efficiency grinding for sapphire substrate can be performed with the two-dimensional vibration grinding technique presented in this paper.