This paper deals with the motions of a high-speed container ship at sea, which is chosen as the prototype of the ships recently built in Japan. The model experiments in regular and irregular oblique waves were performed in order to compare their results with those obtained by the theoretical calculations based on the “strip method”, not only as to longitudinal motions but also lateral motions. The agreements between calculated and measured amplitudes of motions are very good for pitching, fairly good for vertical and lateral accelerations, good for rolling, while the difference is considerable in yawing. The results of the theoretical calculations covering wide range of sea conditions are also represented as the contour maps of the amplitude, by which the effects of ship speed, encounter angle to wave and wave period on severe motions of the ship are discussed. As the results, for example, the cause of the large rolling motion which has been frequently experienced at such high-speed vessels in quartering sea is shown.