Abstract: In order to develop a new heat-spreading material used as heat sink,diamond/Si composites were synthesized by a high-pressure high-temperature(HPHT) method.An investigation was conducted on the effects of diamond particle size,diamond content,siliconizing process,and coating Ti on diamond particles on the density and thermal conductivity of the composites.The results show that mixing large and small size diamond particles,coating Ti and siliconizing process are effective to improve the density and thermal conductivity of diamond/Si composites.The thermal conductivity rises with the content of diamond increasing.When the mass fraction of diamond blended by 75/63μm Ti-coated diamond particles and 40/7μm diamond particles is 95%,the thermal conductivity of the composite sintered at 4~5GPa and 1400℃ by the HPHT siliconizing method can reach to 468.3W·m^-1·K^-1.