In the previous paper, studies on the ultimate strength of stiffened plates with initial deformations were made, and a new analytical method was proposed. In this paper, the effects of initial imperfections due to welding on the ultimate strength are investigated. It is well known that these welding-induced imperfections which inevitably occur during the manufacturing process reduce the compressive strengh of structural elements. Almost all the strength analyses that have been made so far are based on the measured or assumed initial imperfections. Firstly, in this paper, the practical method to predict welding-induced imperfections such as welding distortion and welding residual stress is introduced. Secondly, an analytical method reported in the previous paper is further developed for describing welded plate behavior in compression. Finally, making use of aboved-mentioned results, the ultimate strength of stiffened plates is discussed. The following conclusions are obtained. 1) It is introduced how to predict welding-induced imperfections according to welding conditions. The authors extended their method in order to estimate the ultimate strength of welded plates with these imperfections. Since this method is an analytical one that the ultimate strength can easily be estimated comparing with other numerical methods. 2) The obtained results are checked against available results from numerical analyses and experimental data. It is confirmed that this proposed method is fairly efficient one. 3) The imperfections due to welding reduce compressive strength of plates. Reduction in ultimate strength is most remarkable in case of the square plate having both slenderness ratio b / t √σ _Y / E =1.9 and small intial deformation. For slenderer plates, reduction ratio caused by welding-induced imperfections is approximately constant. For thicker plates, the effect of residual stress on the ultimate strength disappears. 4) The welding residual stress in stiffened plates lowers the panel element strength so that minimum stiffeness ratios (γ ^B _min, γ ^U _min) of stiffened plates becomes smaller than those (γ ^B _min, γ ^U _min) of stress-free stiffened plates.