In order to analyze the nonlinear behaviour of large sized structures such as ships, composed of many stiffened plates, the “rectangular plate unit” and the “stiffened plate unit” are developed in the framework of the Idealized Structural Unit Method which has been developed by the authors. For each unit, the nonlinear behaviour is idealized, failure conditions and stiffness matrices corresponding to the respective failures are formulated. A computer program has been completed and the accuracy of the new units is confirmed in comparison with the results of analysis by the finite element method. The behaviour until ultimate strength of a large example structure is analyzed using the new units. The units are found to be very effective since very short computing time is necessary for the analysis.