Thin walled structures such as ships and offshore structures are composed of many stiffened plate panels. To analyze these structures efficiently, “Idealized Structural Unit Method (ISUM)” was proposed by one of the authors. In the first report, eigen-function and plastic node method were introduced to calculate a plate panel, which exhibits complex behavior especially among some ISUM elements. Plastic node method is not so satisfactory in reference to computation time and accuracy for analysis of long rectangular plates. To improve them, new plate element is developed in this study, using selective eigen-functions for large deflection and sectional yield conditions at each integration points for plasticity. The stiffness of this element is calculated by numerical integration using the trapezoidal integration method. It is satisfactory that the number of integration points is only 5 × 5 for a entire square plate. This method can efficiently and accurately analyse the behavior of rectangular plates with initial imperfection such as initial deflection and welding residual stress. Necessary computation time is very short for any aspect ratios and the accuracy is very high in comparison with FEM. This element is a new efficient rectangular ISUM element.