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We report the properties of the diamond film/alumina composites that were thought of as promising substrate materials for integrated circuits with ultra-high speed and high power. The compressive stress in diamond films formed by hot filament chemical vapour deposition is reduced by implantation of carbon ions into alumina substrates before the deposition of diamond films. It is found that the stress in the diamond films decreases linearly with the increment of the C+ implantation dose. The measurement results of dielectric properties of diamond film/alumina composites show that the coating of CVD diamond films could effectively reduce the dielectric constant of the composite. The carbon ions implantation into alumina substrates prior to the diamond deposition can reduce the dielectric loss of the composite from 5×10-3 to 2×10-3, and can make the composite have better frequency stability. The thermal conductivity of composites could be obviously increased by coating CVD diamond film. The composite has a dielectric constant of 6.5 and a thermal conductivity of 3.98W/(cm·K) when the thickness of diamond film is up to 100μm.