Containerships have remarkably larger hatches as compared with conventional cargo boats, resulting in the reduction of deck rigidity and overall torsional rigidity. Several studies on the torsional rigidity of ship's hull structure having larger hatch openings has been carried out taking the effects of cross decks into considerations. The width of cross deck, however, which resist to the hull torsion has to be limited to the minimum from the economical point of views of container loading and therefore it can not always be easy to fulfil these contradictive conditions successfully in the design of the hull structure. Accordingly, the effectiveness of bulkhead structure against torsion and the effect of the rigidity and numbers of bulkheads on the torsional rigidity of ship's hull are examined both theoretically and experimentally in the study herein presented. At first, it is confirmed from elementary theory and experiments that bulkheads are considerably effective for the torsion of the thin walled beam of U-shaped section whose torsional behavior can be described with bending-torsion theory. Secondly, a method of analysis to calculate the torsional rigidity of the beam which has uniform open section and arbitrary numbers of elastic bulkheads is derived on the basis of the obtained resistive behavior of the bulkheads from a series of experiments on ship's model. Thirdly, the application of this analysis to container ship introducing the equivalent length of the box beam representing the effect of the bow and stern in ship's form is discussed and further researches are made to obtain the more accurate analysis for practical applications introducing the effect of the bow and stern in ship's form directly into the theoretical process of the bending-torsion analysis. Lastly, it is concluded that the effective bulkhead structure will prevent a larger reduction in torsional rigidity of container-ship hull structure.