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 previous reports, eigen-function for large deflection and sectional yield conditions at each integration points for plasticity were introduced to calculate a plate panel, which exhibits complex behavior especially among some ISUM elements. This method can efficiently and accurately analyze the behavior of rectangular plates with initial imperfection such as initial deflection and welding residual stress. Recently researches on structural reliability of ships and offshore structures are actively performed such as reliability design of ship structure, quantifying randomness and uncertainties on response and strength of ship structure, and reliability design assessment of offshore structure. Reliability evaluation of structures is important. In this study reliability analysis of the ultimate strength of ship structural models is performed using this developed element and method. Firstly it is shown that the developed element can be applied very efficiently to ultimate strength analysis in contrast with finite element method. Secondly reliability evaluation for ultimate strength of structural models is performed using the present method of ultimate strength analysis. Local failure modes of the models change as the load increases until ultimate strength state. Numerical analysis such as the present method is only possible to analyze the complex behavior. Since analysis of the reliability of ultimate strength needs analysis of ultimate strength repeatedly, finite element method isn't an effective method to use for reliability analysis of thin walled structures. This method is very effective not only for the analysis of ultimate strength but also for problems of reliability evaluation.