Different interfacial structures of diamond/Cu composites were synthesized by varying the boron (B) content. The microstructural, thermal, and mechanical properties of the composites were investigated. The results showed that diamond/Cu-B composites had a high thermal conductivity of 695 W/(m·K) and high bending strength of 535 MPa attributed to the formation of a micron-scale dentate B₄C interface structure. The dentate B₄C provided dual functions of metallurgical bonding and mechanical meshing for interface bonding. A semi-coherent interface was formed between the diamond and B₄C, where the diamond (111) and B₄C (104) planes were parallel. The micro/nano-scale dentate structure improved the phonon transmission efficiency and bonding strength at the interface.