Fatigue crack propagation behavior near the fusion line between SA508 ferritic steel and Ni-based buttering metal was studied to assess the integrity of dissimilar metal welded zone in reactor pressure vessels. Ni-based filler metal has been used as a buttering or filler metal to weld the ferritic steel to the Ni-alloy or austenitic stainless steel. The J integral value and stress field at the crack tip in a simulated small-CT welded specimen model was calculated by using the commercial FE calculation code to anticipate the effect of the yield strength differences between dissimilar metals. If the Ni-based buttering metal has lower yield strength, which means the decrease of material constraint by the weld metal, the J integral value of the crack tip in the base metal near the fusion line was calculated higher than that of the base metal. The fatigue crack propagation behavior near the fusion line was measured by using the small-CT welded specimens of 5 mm thickness. The relationships between da/dN and )K were measured in the base metal and the HAZ near fusion line. The yield strength of the weld metal including microstructure at the joint can be considered more effective than the material constraint on explaining the behavior of fatigue crack propagation near the fusion line.