Abstract: Carbon diffision in α-γ dissimilar steel welded joint at high temperature was investigated.A mathematical model for the relationship between the thickness of carburized and decarburized layer(H)and the heating temperature and time was established by means of nonlinear regressive analysis method.The research results showed that the maximun thickness of carburized and decarburized layer,which is equal to the result of the former half part of the model,is a function of temperature,and that the latter half part of the model influenced by diffusion coefficient is a function of heating temperature and time.The mathematical model is identical to the test results,so it can be used to estimate the thickness of carbuized and decarburized layer under different conditions.