Abstract: In this paper the division of zones,the formation of fusion zone and the chemical inhomogeneity in γ+α dissimilar steel joints are discussed.The hydrogen distribution in dissimilar steel joints was directly observed by a tritium-labelled autoradiograph.The activity and trap strongly affect the diffusion and distribution of interstitial elements (carbon,hydrogen,etc.) in the fusion zone.Carbon segregates in the fusion zone of weld metal,and hydroges accumulats in the true fusion line.The phenomenon of the lack of carbon and hydrogen occurs in the fusion zone of base metal.The true fusion line labelled by tritium is a heavy boundary layer of about 2~3μm in width.