This paper discusses the experimental and numerical investigation for the ultimate strength of box girder structures. The ultimate strength is one of the most fundamental strength to ensure the structural safety against the applying loads for hull structures. Up to now, therefore, some experimental tests have been carried out utilizing the box girder structures instead of hull structures in order to obtain the valuable knowledge and information about the progressive collapse behavior under the applying loads. In the present paper, the experimental results of the progressive collapse of box girder structures, which are made with consideration for the slenderness ratio in a condition close to real ships of bulk carriers, subjected to pure bending moments by the four points bending test are presented. Also the ultimate strength of box girder structures is calculated utilizing the non-linear finite element analysis as well as the simplified analysis based on the Smith's method, and these results are compared with the experimental results.