In order to obtain high thermal conductivity materials for efficient heat sink applications, diamond particles dispersed CuB alloy matrix composites (CuB/diamond composites) containing diamond as high as 90 vol.% were produced by a high temperature–high pressure infiltration method. The thermal conductivity of the Cu–0.3 wt.%B/diamond composite was measured to be 731 W m−1 K−1. The presence of B4C on the surface of diamond was identified by Raman scattering. The influence of B addition on interface microstructure was clarified to interpret the thermal conductivity enhancement. The results show that B addition can improve the thermal conductivity of Cu/diamond composites produced by high temperature–high pressure infiltration method.