Ultrasonic vibration can be applied for grinding of difficult to cut materials, such as titanium, ceramics, etc., because the ultrasonic vibration in longitudinal direction can cause reduction of cutting temperature and tool wear. In this paper, a ultrasonic tool horn, which can vibrate longitudinally with a frequency of 20kHz, was designed using finite element method (FEM). And the ultrasonic horn was fabricated for ultrasonic assisted grinding. A grinding test was performed in terms of machining parameters, such as grinding speed, feed rate, etc., in order to study effect of ultrasonic vibration in grinding. The design of experiment (DOE) approach was used for an optimal condition of ultrasonic grinding, which can minimize the grinding forces. The grinding forces were measured and compared between the conventional grinding and ultrasonic assisted grinding. In addition, to characterize the effect of work materials, titanium (Ti6Al4V), FCD700 and S45C were used as work materials. From the experiment, it was found that the grinding forces decreased as the ultrasonic vibration power and the rotation speed of spindle increased while the grinding force was reduced as the feed rate increased. In addition, regression model was formulated for obtaining optimal grinding condition. The ultrasonic vibration provided beneficial effects regardless of work materials, especially for brittle material.