Abstract: In this study, a numerical method is presented to study the thickness effect on the residual stress states in butt-welds in 2.25Cr1Mo plates. Using finite element based software ANSYS, coupled thermal-mechanical three dimensional (3D) finite element models have been developed. The finite element models are employed to evaluate the transient temperature and the residual stress fields during welding. Also, in this study the variations of the physical and mechanical properties of the material with temperature have been taken into account. Results show that by increasing plate thickness, the residual stresses increase and the residual stress affected zone (distance with significant amount of residual stress) becomes larger. It has been shown that the longitudinal residual stress in weld axis, change from compressive to tensile stress by increasing plate thickness.