In regard to oil tankers of apploximately 130250 thousand ton deadweight, optimum design of longitudinal strength members is undertaken. Ship size, main proportion, transverse web space and so forth are varied. Rule requirement specified by the ship classification society of Japan is adopted as standard for design. A kind of nonlinear programming techniques, SUMT is used for numerical search. By some numerical results obtained, trends of optimum longitudinal frame space, frame area and plate thickness are examined. Furthermore the influence of using high tensile steel combined with mild steel and that of taking account of fabrication cost are studied. Main conclusions are as follows : I In comparison with actual design, optimum longitudinal frame space, frame area and bottom plate thickness are larger, hence steel weight of midship section decreases by 13%. II As ship size increases, both longitudinal member size and steel weight of midship section increases, however steel weight per cargo space decreases. III When 50 kg/mm² high tensile steel is used combined with mild steel, steel weight decreases by about 12%, while steel cost remains almost unchanged.