Diamond/Ag nanopaste has been prepared by mixing Ag nanoparticles, silver-plated diamond particles, and an organic solvent system. Then, we characterized and studied the sintering properties of diamond/Ag nanopaste. Meanwhile, the impact factor of sintering temperature on the sintered microstructure of the diamond/Ag nanopaste on direct bonding copper (DBC) substrate was investigated. The influences of sintering temperature on the shear strength of diamond/Ag nanopaste connection layer for attaching silicon chip on the silver-plated copper substrate were analyzed. The results show that silver plating on the surface of the diamond can significantly strengthen the bonding ability between the diamond and nano-silver. When the pressureless sintering temperature increased from 150 °C to 350 °C, the porosity of diamond/Ag nanopaste decreased from 29.8% to 8.9%, and the sintered diamond/Ag joint with uniform and dense connection layer showed the maximum shear strength of 15.26 MPa as sintering at 350 °C.