For the analysis of microstructure in the transition zone occurring in weld metal near base metal in welding dissimilar metals, Transition Zone Transformation (TZT) diagram was proposed, which gives the distribution of liquidus, solidus and transformation temperatures in the transition zone. As an example, the TZT diagrams in the transition zone between 2・1/4Cr-1Mo steel and type 309 overlaid metal were made on the basis of Cr and Ni distributions measured in the transition zone. Further, the combination of the TZT diagram with the temperature distribution during overlaying yielded the elucidation of formation mechanism of austenite coarse grain-boundary as the site of hydrogen disbonding. The proposed mechanism was confirmed experimentally, and could be stated as follows: The austenite (γ) grains at the fusion boundary in HAZ formed in δ→γ transformation during cooling are going to grow into the overlaid metal. Before that, however, other γ grains have already nucleated and been growing in the transition zone by the reaction of liquid→liquid+δ→liquid+δ+γ during solidification. Therefore, when the γ grains from HAZ grow only a little into the overlaid metal, both the γ grains from HAZ and in the overlaid metal collide with each other in the transition zone, and this collision makes γ grain-boundary parallel to the fusion boundary. This γ grain-boundary shifts a little accompanying the disappearing of δ during cooling, and the zone between the γ grain-boundary and the carbide layer formed after PWHT is regarded as so-called γ coarse grain.