Composite of diamond particulates/copper was fabricated through diffusion bonding with a thin Ni-foil as interlayer via a spark plasma sintering technique. Then the formed bonding interface was characterized by means of SEM+EDS, XRD, universal testing machine and thermal conductivity meter, in terms of its microstructure, fracture morphology, shear strength, and thermal conductivity. Meanwhile the effect of process parameters on the microstructure and thermal conductivity of the bonding interface was investigated, and the variation of the interface behavior during diffusion bonding process was also studied by the first principle calculation. The results showed that the formed interface presents excellent metallurgical bonding between the diamond particulates and the Cu. The presence of the thin Ni foil interlayer significantly reduced the formation of voids and holes at the diffusion-bonded interface. The diffusion of Ni led to the formation of an α single-phase Cu-Ni solid solution and the production of the Ni₃C phase. The phonon and electron density of states maps of the Cu/Ni-interface showed that the thermal conductivity of the binary interface was better than that of the diamond/nickel interface, and the high proportion of Cu/Ni-interface connection at the interface was an important factor in achieving high thermal conductivity. The spark plasma sintering induced diffusion joint interface presents thermal conductivity up to 703.83 W/(m·K), while the infinite solid solution of Ni-Cu, as well as the generation of carbide is conducive to the thermal conductivity of the joint.