When plate structures are constructed by welding, such troublesome matters as welding residual stresses and welding deformations sometimes occur. Among welding deformations, the buckling deformation which occurs during the fabrication stage of thin skin panel structures is one of the most important problems. It is because the magnitude of this buckling deformation is much larger compared with other kinds of welding distortions. In spite that many investigations have been conducted on this problem, it cannot be said that practical conclusions are obtained in shipbuilding industries. In order to find out the practical prevention method of this type of deformation, a basic analytical method considering the following problems is studied in this paper. 1) buckling deformation due to butt welding condition at which buckling occurs is examined. 2) lateral deformation due to fillet welding not only in-plane shrinkage but also angular distortion is considered. At first, buckling strength of a rectangular plate with one dimensional residual stresses subjected to compression is examined. The relation between buckling stresses and plate dimensions b / t (plate width/thickness) for simply supported plates (both mild steel and Al alloy) concerning constant heat input is obtained. The results of the calculations show the plate dimension b / t at which the plate undergoes buckling due to welding residual stress. Secondary, lateral deflection of a plate due to fillet welding is analysed based on Rayleigh Ritz method. The inherent strain distribution ε^* which is used during calculation is determined according to welding heat input Q / t . As for angular distortion of a panel restrained by stiffeners it is considered as an initial imperfection. On the above bases, an approximate method for calculating the deflections of flat or initially imperfect plate subjected to inherent strain is proposed. Critical heat input ( Q / t ) _cr above which the plate cannot remain plane is obtained for square steel plate of 1000 × 1000 mm and 500 × 500 mm. Experimental results for those plates are in good agreement with the theoretical ones.