Abstract: A thorough understanding of the mechanism of nucleation and growth particularly at the early stage, is very important for the preparation of high quality diamond films. Using a semi-empirical quantum mechanical method (PM3), we calculated the potential barriers of reactions of chemical vapor deposition (CVD) diamond films on Si(111) substrate. Applying methane and acetylene as precursors, we investigated the abstracting of hydrogen and adding of methyl and acetyl radicals at the growth surfaces of Si(111) and diamond(111). It shows that, for both interfaces, the barriers of abstracting hydrogen are much lower than those of adding growth radicals. However, the barrier when acetyl is the precursor is higher than when methyl for both the diamond(111) and Si(111), respectively. It is suggested that only when the C≡C bond of acetylene molecular breaking can it be added to the growth surfaces.