When a plate structure is externally loaded, it is considered that structural elements such as stiffened plate and panel are usually subjected to combined stresses. In order to estimate the ultimate strength of these structural elements, as a basic study, a square plate subjected to compression and shear is treated in this paper. As a practical example of such elements in ship structure, horizontal girders, a part of longitudinal bulkhead and so on can be cited. In the previous papers, the authors proposed a new analytical method for evaluating the ultimate strength of a stiffened plate subjected to compression. This simplified method in which numerical and experimental results were taken into consideration was proved to be fairly efficient. In this paper, this method is extended for a problem of estimating the ultimate strength of a square plate under compression and shear. This extended method consists of two parts. The first one is elastic analysis in which Galerkin's method is adopted. The second one is plastic analysis in which collapse mechanism is assumed. In both of them large deformation theory is considered. The analyses were carried out for a problem of a plate subjected to various combinations of compressive and shear loading. In addition, the influence of initial imperfections on the ultimate strength was clarified. The following conclusions are obtained. 1) For a plate whose edges are kept straight under pure shear, it can be said that the maximum shear stress which corresponds to ultimate strength equals to τ _Y . 2) The ultimate strength of thick plate without initial distortion is not influended by types of load in combination. With the decreasing thickness of plate, in the range where compressive load is dominant, the ultimate strength becomes smaller. 3) On the other hand, the influence of initial deformations on the ultimate strength becomes remarkable with the increase of plate thickness and share of compressive load.