Abstract: In order to research the influences of ageing factors on carbon migration in dissimilar metal weldments, the electron probe was used to inspect carbon distribution at the interface of ferritic basic metal side. The least squares fit for experimental datum of carbon migration and the possibilities of the state identification by least square method were discussed. Three series of tests were carried out and the experimental and fits results were analysised on the basis of phenomenology. on the basis of analysising of test results, we can discover: the more seyvicing temperature and time are, the more carbon migration, width of carbonizating layer and decarbonizating layer are. It is im possible that they can be imitated in the least square method.