The ultimate strengths of compressive steel plates simply supported on three sides and one side free (outstanding plates) were numerically investigated by means of combining FE elasto-plastic analysis and Monte Carlo simulation considering the statistical data for variability of the initial displacement and residual stress. From this study, the following results were obtained: (1) a standard deviation of the ultimate strength tends to increase in the region of the with-thickness ratio parameter R ≅ 1.0; (2) in the region of R < 0.7, the effect of the residual stress on the ultimate strength is very small; and (3) considering the statistical data for variability of the initial displacement and residual stress of the plate, a rational design formula can be proposed.