A series of buckling eigen-value calculations and elastoplastic large deflection analyses of simply supported rectangular plate with cutout subjected to in-plane loads has been performed. Firstly, advanced simple formulae in practical use for estimation of elastic buckling strength have been derived based on the results of eigen-value calculations. Secondary, relationship between the ultimate strength and the buckling strength evaluated by the existing plastic correction methods has been clarified. By considering the yielding in the effective cross section with cutout, new plastic correction formula for estimation of plastic buckling strength of rectangular plates with cutout has been proposed. Finally, based on the results of elastoplastic large deflection analyses, simple formulae for estimation of ultimate strength of rectangular plate with cutout subject to longitudinal thrust have been derived.