In this study, thin-plate bending theory using the assumption of small deflection has been adopted for a two-dimensional analysis of the thermo-elasto-plastic behavior of plates in welding to overcome some difficulties arising from a fully three-dimensional approach. For simulating the temperature gradient through the plate thickness, the concept of a layered plate has been proposed, and the thermo-elasto-plastic behavior of the plate during GMA bead-on-plate welding analyzed by using the finite-element method. It was revealed that the suggested method can predict the trend of welding distortions quite satisfactorily.