Abstract: The thermal conductivity of diamond particles reinforced copper matrix composite as an attractive thermal management material is significantly lowered by the non-wetting heterointerface. The paper investigates the heat transport behavior between a 200-nm Cu layer and a single-crystalline diamond substrate inserted by a chromium (Cr) interlayer having a series of thicknesses from 150 nm down to 5 nm. The purpose is to detect the impact of the modifying interlayer thickness on the interfacial thermal conductance (h) between Cu and diamond. The time-domain thermoreflectance measurements suggest that the introduction of Cr interlayer dramatically improves the h between Cu and diamond owing to the enhanced interfacial adhesion and bridged dissimilar phonon states between Cu and diamond. The h value exhibits a decreasing trend as the Cr interlayer becomes thicker because of the increase in thermal resistance of Cr interlayer. The high h values are observed for the Cr interlayer thicknesses below 21 nm since phononic transport channel dominates the thermal conduction in the ultrathin Cr layer. The findings provide a way to tune the thermal conduction across the metal/nonmetal heterogeneous interface, which plays a pivotal role in designing materials and devices for thermal management applications.