Stiffened plate is a fundamental component of ship hull. The main purpose of the study is to present an analysis method which may be easy but effective to get a good estimation of stress-deflection behavior of stiffened plates. A rectangular plate subjected to uni-axial compression is the main target in the first report. The proposed method is called “AEPM (advanced elastic plastic method)” as it is based both on the elastic large deformation analysis and on the plastic analysis. AEPM has been found to be effective in the region not only pre-ultimate but also post-ultimate by taking advantage of a refined equation derived from FEM data base as a regression curve. It has been verified by comparing its results with FEM that AEPM keeps as good precision as FEM and it is available for the wide range of structural characteristics including plate slenderness ratio, residual stress, initial deflection and plate aspect ratio.