The compressive stress in the diamond films formed on alumina ceramics can be reduced by implantation of carbon ions into alumina substrates before the deposition of diamond films. After carbon ion implantation, there is no new phase identified as interlayer by comparing the x-ray diffraction (XRD) patterns of the un-implanted and implanted substrates. In addition, the stress in the diamond films decreases linearly with the increase of the C+ implantation dose. This is because the implanted ions settle in the interstitial position and induce the residual compressive stress in the alumina lattice. This compressive stress, when the diamond films deposited and cooled down to room temperature, will be released and partly offset the compressive stress in the diamond films.