High thermal conductivity Cu/diamond composites were fabricated using an electrodeposition technique. The electrodes were oriented horizontally, and the cathode was located at the bottom of the plating bath. Diamond particles (10-230 mu m) were first precipitated on the cathode substrate, and then copper was electrodeposited on the substrate to fill the gap between the precipitated diamond particles, which resulted in the formation of a Cu/diamond composite. The deposition behavior of the copper was electrochemically investigated, and the current densities of copper deposition under galvanostatic conditions were estimated. The current densities for the substrate with diamond particle layers were 4-10 times higher than the current density for the substrate without diamond particle layers, which led to undesired hydrogen evolution. Cu/diamond composites were formed under potentiostatic conditions without hydrogen evolution, and the resultant composites had compact morphologies. A specimen containing 49 vol% diamond particles with a mean diameter of 230 mu m had the highest thermal conductivity of 600 W m(-1) K-1, which is 1.5 times that of pure copper (ca. 400 W m(-1) K-1). Graphic High thermal conductivity Cu/diamond composites were fabricated by electrodeposition under a potentiostatic condition without the evolution of hydrogen gas.