Ship and bridge structures mainly consist of stiffened plates. Studies on the ultimate strength of these stiffened plates were made by using the concept of the effective width. (Kármán, Yoshiki) Because of a recent great progress in application of finite element method in structural analysis, large-deformed elastic-plastic problems could be solved. The obtained results yield several significant informations. Since there are, however, many types of stiffened plate structures, it is difficult to carry out direct calculations (such as F. E. M.) for each structure. In order to estimate the ultimate strength of stiffened plates, a new analytical method is presented in this paper. The proposed method is compound of two parts. One is elastic analysis based on RayleighRitz method, the other is plastic analysis in which collapse mechanism is considered. Both analysis treat large deformation. The following conclusions are obtained. 1) Post buckling behavior and the ultimate strength of a square plate subjected to compressive load can be obtained by using a newly proposed analytical method. These results show fairy good agreements with those based on the concept of effective width and those calcul ated by F. E. M. 2) Relationship between the ultimate strength of a stiffened plate and its stiffener height as well asγ ^U min, (minimum stiffeness ratio of a stiffener against ultimate strength) can be also analysed by this method. These results fairy agree with those calculated by Ueda et al. using F. E. M. 3) This newly proposed method makes use of analytical solutions so that the ultimate strength of stiffened plates can be easily estimated comparing with other numerical methods.