Electronic structure calculation was carried out for the hydrogen-terminated carbon cluster designed to model the {111}-oriented diamond surface, Cd (111).Using LDA method with local generalised gradient corrections, the subject of the calculation included the adsorption of single-carbon species and double-carbon species on an activated diamond surface. The results showed that: (1) Single-carbon species will more likely result in {111}-oriented CVD diamond film growth than double-carbon species, however, the role of double-carbon species should not be ignored; (2) The adsorption of single-carbon species on activated site makes the system more stable than that of double-carbon species. (3) Our computation result of CH3 adsorption on growth surface is in good agreement with that acquired both theoretically and experimentally.