An oil-tanker “Seiho Maru” was built at Kawasaki Dockyard in January of 1952. The authors considered that the lines of such a large and full ship as “seiho Maru” should be determined from the point of view of arrangement, construction and equipment as well as propulsive performances. Therefore, in this report, comparisons were made for two kinds of lines on basis of arrangement, construction and etc., one of which was the original form of “Seiho Maru” and the other modified form. Then, for three kinds of lines, adding a thirdform to the former two lines, comparisons were made on a base of propulsive performance. It will be shown from their results that a certain lines may be determined so as to have a superiority in every point of view.