In the early morning of January 2, 1997, a Russian tanker, Nakhodka, had broken into two in the Sea of Japan, and the coast of Japan was seriously polluted by the spilled heavy oil. Following this disaster, the committee for the investigation on causes of the casualty of Nakhodka was established. In this paper, the characteristics of structural strength of Nakhodka at the casualty are described, which could have been known through the research activities and discussions in the committee. At the beginning, structural characteristics of Nakhodka are described, and the thickness reduction of structural members are estimated based on the measured results. Then, the ultimate longitudinal strength of Nakhodka at the casualty is evaluated applying the so-called Smith's method, and the possibility of overall collapse is discussed. The mechanism of fracture at the bottom plating is also discussed according to the scenario based on the observed fracture surface of the cross-section. At the end, a numerical simulation of the break-up of the hull girder is performed. It is shown that buckling/ plastic collapse took place at the deck plating near Fr. 153, which was followed by the successive collapse of side shell plating. At nearly the same time, the bottom plating fractured just in front of the transverse bulkhead at Fr. 153.