Initial imperfections such as residual stress and deflection are inevitable for steel structures fabricated by welding. The estimation of the magnitude of these imperfections and their effect on the strength of structures are indispensable to rational design. Firstly, the magnitudes of initial imperfections both in panels and in stiffeners are estimated according to and modifying Sato's empirical formula. Secondly, a refined method of analysis is introduced, in which the original method is so improved as to take into account arbitrary mode of imperfections in a stiffener. Finally, the effects of initial imperfections on the reduction of ultimate strength of stiffened plates are discussed by comparing the calculated results. Conclusions obtained in this study are : (1) A simple but useful method to predict residual stresses and initial deflections both in panels and in stiffeners due to heat input is introduced, and it is found to give satisfactory results as compared to the data obtained from scale models and existing structures. (2) The refined calculation method to derive the ultimate strength on the basis of elastic large deflection analysis and plastic analysis are used in a parametric study and the results are validated by comparing with FEM analysis. It is shown that the effect of residual stress is underestimated and that of initial deflection a little over emphasized. (3) The reduction of ultimate strength caused by initial deflection in stiffeners is significant in the region of β<2, but the effect of residual stress in stiffeners is little in almost all the region of β.