In this paper, described are some aspects of an experimental test on the double bottom structures of a 58, 500 DWT ore/oil carrier. The test was conducted under light condition of afloat ship, where the load was applied on the double bottom structures by filling sea water into the cargo hold.Measurements were made on the deflections of the double bottom by using three different methods simultaneously, namely, “scale and camera method”, “microscope and dial gage method” together with “scale and transit method”. At the same time, an extensive stress measurement was carried out at more than 800 measuring points of the structural members including floor plates, girders, tank top plate and longitudinals as well as hopper and double hull constructions at ship side, and the hopper and cofferdom constructions of the transverse bulkhead.Theoretical analysis was made on the mechanical behavior of the double bottom structures, by taking into considerations both flexural and torsional constraints of the hoppers, shoulder tanks and the bulkhead structures.Discussions were also made on the deformations of the longitudinals, pointing out that these members do not bend as a unit beam with longitudinal girders as have been assumed in the ordinary bending theory of beam.It was also found from this study that the shear deflections of the double bottom structures are usually of almost same magnitude as that of the bending deflections, due to the existence of comparatively high shearing stresses in longitudinal girders and transverse floors at their ends.In general, the theoretical predictions have shown a fairly good agreement with the experimental results, and thus it is concluded that a rational estimation on the strength of double bottom structures can be made in accordance with this analytical method, of which the calculation program is prepared for the use of initial design of ships.