Diamond-copper composites were prepared by powder metallurgy, in which the diamond particles were pre-coated by magnetic sputtering with copper alloy containing a small amount of carbide forming elements (including B, Cr, Ti, and Si). The influence of the carbide forming element additives on the microstructure and thermal conductivity of diamond composites was investigated. It is found that the composites fabricated with Cu-0.5B coated diamond particles has a relatively higher density and its thermal conductivity approaches 300 W/(m·K). Addition of 0.5%B improves the interfacial bonding and decreases thermal boundary resistance between diamond and Cu, while addition of 1%Cr makes the interfacial layer break away from diamond surface. The actual interfacial thermal conductivity of the composites with Cu-0.5B alloy coated on diamond is much higher than that of the Cu-1Cr layer, which suggests that the intrinsic thermal conductivity of the interfacial layer is an important factor for improving the thermal conductivity of the diamond composites.