Abstract: The diffusion welding behavior of single crystalline Cu to single-crystalline α-Al₂O₃ with a Nb film interlayer and the joint microstructure properties were studied by TEM, SEM/EDS analyses and four-point bend testing. The Nb film interlayer deposited by electron beam evaporation on the ceramic side prior to diffusion welding was found to be polycrystalline and fiber-textured after diffusion bonding, with the close-packed plane (110) being parallel to the (0001) basal plane of α-Al₂O₃. The fracture energy of Cu/α-Al₂O₃ diffusion-welded joints was greatly increased by the introduction of the thin film Nb interlayer whereas diffusion welding can be performed at a relatively low temperature (900℃). The increase in fracture energy of the joints with Nb film interlayer was attributed to the strong adhesion of Nb to α-Al₂O₃ combined with larger plastic deformation in the metal side during fracture. TEM observations showed large amounts of dislocations existing at the interfacial regions.