In this paper, the ultimate strength of stiffened rectangular plate. under any combination of compressive, inplane bending, shearing and lateral pressure load are studied. The problem has been analyzed by means of FEM or FSM, but these analysis methods take much larbor and time so that a handy analysis method is necessary at the initial design stage. For this reason, analytical method for easy parameter survey is presented. This method consist of two parts. One is elastic large deformation analysis which takes account of the plastification of the stiffener by simplified way. The other is plastic analysis of the plate without stiffener in which collapse mechanism is assumed. The analysis are carried out for various problems of stiffened plate and the following conclusions are obtained; 1) The ultimate strength of square plates under uniform compression with symmetrical stiffener and one-side stiffener evaluated by this method well agrees with the results of FEM analysis and in the case of stiffened rectangular plates the results also well agree with the experimental results. 2) The analysis of rectangular plates under uniform compression shows that the mode which gives the minimum buckling load does not always give the minimum ultimate strength. This phenomena is subjected to the value of the initial deformation and aspectratio. 3) The present method for the analysis of the ultimate strength of stiffened plates under uniform compression and lateral pressure is available when lateral pressure is not so prominent, however when lateral pressure is predominant the present method gives much conservative value.