Abstract: Diamond particles were roughened with Pr₆O₁₁ as the etching agent, and the diamond/copper (boron) composites with the diamond volume fraction of 60.0% and the boron volume fraction of 0.3% were prepared by spark plasma sintering. The effects of diamond surface modification and matrix boron alloying on the thermal conductivity of the diamond/copper composites were investigated by the experiments, thermal flux simulations, and phonon density of states. The results show that, the roughness of the diamond interface increases the contact area; the B₄C phases appear after the sintering when the boron element is added in the matrix, and the formation of the B₄C phases improves the bonding state of the diamond‒copper interface. The interaction of diamond roughness and matrix alloying can effectively reduce the interfacial thermal resistance, optimize the efficiency of heat flux transfer, and improve the thermal conductivity of composites. The thermal conductivity of diamond/copper composites increases from 421 W·m⁻¹·K⁻¹ to 598 W·m⁻¹·K⁻¹, increasing by nearly 42%.