The butt welding of 45 carbon structural steel has been analyzed using a nonlinear transient finite element (FE) model. With numerical analysis technology, welding heat transfer and thermal elasto-plastictity have been progressed. Also the effects of welding seam growth with heating source movement have been studied using the birth and death technique. For this purpose, a 3-D structural and thermal direct-coupled solution has been used to obtain the temperature changing curves and residual stresses distributions. In addition, the variations of physical properties of material with temperature have been taken into account. Results indicate that the heating speed of welding is rapid, and the distributions of residual stresses are not symmetrical along the welding seam’s centerline, and the residual stresses are change along the welding direction and also in the plate-thickness. It has been shown that use of 3-D direct coupled-field and nonlinear transient model will lead to more accurate and realistic results.