Abstract: The relationship between the behaviour of the fatigue crack propagation and the microstructure in the austenitic-ferritic dissimilar metal joints has been studied based on the three-point bend specimens, and the data of crack propagation velocity da/dN have been obtained. The results show that the martensite layer has the effect of resistance to the fatigue crack propagation in the Cr25-Ni13/13CrMo44 dissimilar metal joints. The path that the fatigue cracks propagates is in the overheated zone in HAZ, which is weak in toughness. The crack propagation is mainly controlled by the toughness of the microstructures.