A bulk carrier in the fully laden condition suffered disastrous structural damage in rough seas in the North Pacific Ocean off Japan where two systems of swells were dominant; one was regarded as a regular wave train of 89 m in height, and the other is irregular one of significant wave height 46 m. Her fore body before the mid-length of No.1 Cargo Hatch was bent upwards and was seperated from the main body soon. Forty days later, she sank during being towed to Wake Island. The process of the disaster is analyzed in the present paper from the viewpoint of hydro-elasticity on the basis of her captain's report on this event. It is concluded that the disaster was caused by buckling of the upper deck due to slamming impact in an unlucky superposition of the two systems of swells.