When structures are constructed by welding, heat conduction brings welding deformation. This is accompanied by complicated mechanical phenomenon such as material nonlinear and geometric nonlinear behavior. Hitherto, the research of welding deformation has been accomplished by an analytical method and experimental data in Korea. In this paper, the computer program by F.E.M.(finite element method) which could analyze the deformation of thin plate considering phenomena(both material and geometric nonlinear behavior) has been developed and verified. The production mechanism and characteristics in the welding deformation of plate are studied by the results.