It is reported that several cargo ships have suffered from buckling failures of deck or bottom plates stiffened transversely. The ultimate compressive strength of rectangular plates is studied by analyzing a simplified model which clarifies the effects of yielding on the strength. Approximate formulas for prediction of the buckling strength are proposed in terms of the aspect ratio and the ratios of yield stress to elastic buckling stress and initial deflection to thickness. The accuracy of the formulas is verified by the detailed analysis using the combined technique of the finite-element and energy methods.