When designing special ships such as LPG carriers, container ships, etc., it is necessary to estimte the maximum rolling angle at the initial stage of the design. For this purpose, we usually use the approximate formula √<(138rs)/N> stated in the Safety Regulation. However, it seems this formula cannot give an accurate estimation for large vessels with long rolling periods, since it is based on the resonance in beam seas. As for ship motions in oblique waves equations were demonstrated by Yamazaki and Fukuda. But they are too complicated to be used at the initial stage of the design. For this reason the authors attempt to introduce an approximate formula for calculating rolling angles in oblique irregular waves which can be easily adopted for the present purpose.