In this paper, thermo-mechanical behavior of the welding process was analyzed to determine the effect of preheating on the distortion magnitude and distribution in dissimilar joints. By using a verified finite element model, an efficient user subroutine was developed to consider the effects of phase transformation. In order to verify the model, experimental data for dissimilar joints, obtained by CMM measuring device method, were utilized. Good agreement was observed between the finite element and experimental data. The results indicated that the developed computational method is an effective tool to predict the distortion of dissimilar weld joints. The present finite element model was developed in a butt-welded plate to consider the effect of the preheating. It was observed that the increasing of the preheating temperature magnitude of distribution decreased on the carbon steel and stainless steel sides. Moreover, by increasing the preheating temperature more than special temperature range the preheating has no meaningful effect on the distortion on the both side of joint. Whatever, the preheating temperature become asymmetric in dissimilar joints, effect of the preheating on the distortion reduction is negligible on the carbon steel side but it has important effect on the stainless steel side. By increasing the preheating temperature invariably the carbon steel side has distortion of welding less than stainless steel side because of that’s high thermal coefficient.