In general, a deck plate of a ship hull is subjected to inplane tensile and/or compressive load due to longitudinal bending. When the strength of a deck plate is considered, the contribution of the plate elements subdivided by stiffeners and girders are very important, and the accurate evaluation of their compressive strength is essential. In this report, based on the results obtained in this series of research, the authors have developed a simple estimation method of the ultimate strength with a limited amount of information on the kind of steel, sizes of deck plate, welding condition, etc., which does not require complex calculation such as FEM. The main results obtained in this study are as follows. (1) A simple estimating method of welding deformation and residual stresses produced in a deck plate is proposed. The calculation can be performed by using the yield stress of material, welding condition, etc.. The estimation is very accurate compared with the actual measurement. (2) In order to describe the influence of actual complex initial deflection, the effective coefficient of initial deflection is theoretically determined for the standard initial deflection which was idealized from the actually measured initial deflection. The maximum initial deflection influences the ultimate strength fully for thick plates and less than a half for thin plates. (3) The ultimate strength is evaluated from the ultimate strength formulae proposed in the previous report, taking into account of the effective coefficient of initial deflection. The estimated ones are very accurate in the comparison with those obtained by direct FEM calculation to the actual panel.