Cu/Si composite is an ideal material for electronic packaging owing to its excellent thermophysical and mechanical properties. Especially, its high thermal conductivity can fulfill the requirements of quick elimination of heat of high power devices. However, because of the severe diffusion and reaction between Cu and Si, the Cu–Si compound replaces the Cu and Si phases during the powder metallurgy fabrication at elevated temperature. Therefore, the crucial issue of Cu/Si composite fabrication is to control the Cu–Si diffusion and reaction. In this paper, the Cu/Si composites were fabricated using pretreated films on Si powder formed in Al₂O₃/TiO₂ sol as a diffusion barrier to prevent Cu–Si reaction. The phases, microstructures and properties of Cu/Si composites were investigated. The results indicate that Cu/Si composites on which Si powders are pretreated by sol–gel are primarily composed of Cu, Si, and a few Cu₃Si phases. The hardness of the composite is 147HV0.1, and the thermal diffusivity at room temperature is 26.4 mm²/s. Cu and i atoms diffuse via. the defects ifilm and react to form Cu–Si compound in local regions at Cu/Si interface during sintering. However, oly Cu3Si phase is detected in the composite on which the Si powder s not pretreated, and no Cu or Si trace is found. The hardnss is as high as 399HV0.1, but the thermal diffusiviy at room temperature is only 3.0 mm²/s. Thrfore, sol–gel pretreatment on Si powders can effecively reduce the Cu–Si reaction and protect the Cu and Si phases in composites so as to elevate the thermal conductivity.