In the designing of a giant tanker, the length of oil tanks and the distance between bulkheads are among the most important items, because they have significant effect on the motion of cargo oil and the dynamic forces on bulkheads. In the present investigation, the above mentioned effects were experimentally studied in the case where a model tank is given a transient pitching motion. This test condition was considered to most satisfactorily simulate the actual condition of an oil tanker which goes through irregular waves in a rough sea. The effects of the variables such as the motion given to the tank, the level of the oil and the shape of bulkhead also were studied. From the test results, formulas for the design of bulkheads in view of dynamic forces effected thereupon were obtained, and they were compared with the regulations of the N. K. Rule, It was found that the formulas given by the N. K. Rule are reasonable for the design of watertight bulkheads of a ship which is approximately 100m in length while they are unreasonable, as they overestimate the effect of dynamic force, for the design of watertight bulkheads of a ship which is longer than 100 m as well as for the design of swash bulkheads of a ship of any length. In view of the dynamic forces by motion of cargo oil, longer ship is more favorable in adopting a long tank. Swash bulkhead may bring out merely little merits in view of the hull weight reduction resulted from the reduced dynamic forces on the bulkheads. They are to be taken into consideration in view of keeping the transverse strength for hull structure.