The impact pressure acting on bow of large and full ship may be predicted by the following considerations. The ship motions are mainly due to the waves longer than ship but scarcely affected by the shorter waves. Therefore, the ship cruising in the ocean is oscillated violently by the swell (longer waves) and the velocity of the bow motion relative to wave surface becomes very large. Impacts on bow occur when short waves are superposed to the longer waves and collide with such oscillating ship. If the angle between the stem and the wave surface of the short waves is small, large impact pressure is produced. The slope of stem and bluntness of bow are considered to be main parameters controlling the amount of pressure and its frequency. At Nagasaki Experimental Tank, impact pressures on the model are measured in shorter waves superposed on longer waves or using models oscillated with frequency and amplitude corresponding to the condition in longer waves.