Abstract: In the austenitic-ferritic dissimilar metal welded joints the content of alloying elements in the transition zone varies continuously from HAZ to weld. Due to the low level of Ni content, a martensite layer is formed in this zone during the welding process.The Charpy impact test made before indicated that the martensite layer was the weakest zone in toughness in the joints. In this paper, the simulating lest, in which specimens were prepared by casting steels in accordance with the compositions in the martensite layer respectively,was conducted to evaluate the fracture toughness of the martensite layer in the homogeneous structure. The experiments were alse made to investigate the variation in toughness in the different regions of the joint.The results show that the weakest region in toughness in the dissimilar metal joints is not the martensite layer but the overheated zone in the HAZ. The fracture in the overheated zone is caused by the coarse-grained bainite, and it appears as a quasicleavage fracture; however, in the martensite elayer, it appears as a tear fracture.