In this study, stiffened plates under biaxial compression are analyzed by using the commercial FEM software, LS-DYNA, and the collapse behavior is investigated. Firstly, the decrease of effective width of the ship bottom plate is observed and compared with the formula of CSR²⁾. Considering the results of FEM analysis, a new formula to estimate ultimate strength of stiffened plates considering biaxial compression is developed. Secondly, the method to estimate ultimate strength of hull girder considering the biaxial compression of ship bottom is proposed by considering the new formula and modifying the conventional formula of the hard corner element. Finally, hull girder strength considering biaxial compression that is estimated by the proposed method is compared with the FEM analysis of mid ship model to show the validity of the proposed method.