Abstract: The transmission electron microscopic analysis (TEMA) was used as primary technique to show the martensite structure at weld bond of dissimilar-metal joints. In order to determine the distribution of the martensite start points (Ms) in the martensite layer, the simulating test was also made. The experimental results show that the structure of martensite layer in weld bond is dislocation martensite and that the marlensite start points vary with the distance from fusion line, which is controlled by composition gradient. Based on these results, the model which could describe the formation of martensite structure was established in detail