This is the first report of our research about the estimation of the ultimate strength of wing tank structures of tankers and ore carriers under wave pressures. A transverse ring of the wing tank structure with a single strut is approximated as a simple frame structure. The collapsing phenomena of this frame structure are analysed and its collapse loads are calculated under the various values of the factor which have influence upon them, such as the dimensions of frames, the local buckling strength and the full plastic moment of the section of frames under bending force, the buckling strength of strut under axial force, the support condition of full structure, ore pressure and so forth. Fairly good agreement is found between the calculated values of collapse loads and the measured ones obtained by the experiments carried out recently by Shipbuilding Research Association of Japan.