Diamond/copper (Cu) composites are fabricated using a mixture of titanium (Ti)-coated diamond and composite of Cu–Ti powders, which is proposed to improve interfacial bonding between diamond particles and Cu matrix for diamond/Cu composites. Suitable coating and composite of Cu–Ti powders in the sintering process are optimized. The microstructures, structure of the interface, thermal properties, and relative density of diamond/Cu composites are investigated. Results show that the interfacial bonding of composites could be strengthened. The relative density of diamond/Cu composites is 99% in a volume fraction range of diamond between 40% and 65%. Moreover, 50vol% diamond/Cu composites sintered at 1000°C for 5min exhibit a thermal conductivity as high as 630W/(mK). This value is approximately 80% of the theoretical value. A high thermal conductivity is achieved because of the formation of the diamond/TiC/Ti/CuTi/Cu structure at the interface between diamond and Cu.