This paper deals with precautions needed for an accurate numerical analysis of welding residual stress and deformation. By comparison of numerical results with experimental data, the precautions of the thermal-elastio-plastic FEM are investigated. The bead-on-plate welding was selected as the first object of study, because the boundary conditions does not change with the moving of heat souce. The obtained precautions are as follows : The element division is determined by the size on the welding direction of mechanical melting point and the size of inherent area calculated by the fix-bar model. The yield strength in the area of inherent strain and the coefficient of linear expansion in the vicinity of mechanical melting point are needed.