Welded joint between different Cr-Mo steels are used in furnace tubes in petrochemical plants. A carbon migration from low Cr side to high Cr side in joint is often observed, which leads to the formation of decarburized zone and affects the performance and aervice time of the welded joint. Inthis paper the carbon migration between 9Cr1Mo and 15CrMo steels was studied. Accelerating experiments were performed. The relation between decarburized zone width and temperature and time of service was proposed to describe the influence of carbon migration on structural performance.