A novel pretreatment enhancing diamond nucleation has been developed for diamond growth over a large area using a magnetoactive microwave plasma chemical vapor deposition method. After the predeposition of carbon films on Si(100) substrates using CH4/CO2/He gas mixtures, diamond films with high nucleation densities were obtained after a subsequent 2 h growth process commonly employed using a CH4/CO2/H2 gas mixture. In the present study, especially, the effect of CO2 concentration in the CH4/CO2/He gas mixture in the pretreatment process has been examined on the carbon film growth. The results show that the diamond nucleation with densities as high as ∼109/cm2 was attained for small CO2 concentrations of 1%–2% during the pretreatment process, while no successful enhancement was enabled for Si substrates pretreated at high CO2 concentrations beyond 3.7%. The structural property of the predeposited carbon films significantly influenced the diamond nucleation. This was evidenced by in situ data of optical emission spectroscopy and quadrupole mass spectroscopy during the pretreatment process, as well as by ex situ data of morphology and composition of the specimens. The volume density of the carbon films obtained after the pretreatment was maximized at a CO2 concentration of 1.9%. Th- e bonding nature of the carbon atoms deduced from the related Raman scattering spectra apparently changed with CO2 concentration. The role of the predeposited carbon films is discussed in relation to etching and agglomeration phenomena during the subsequent diamond growth process. © 2000 American Institute of Physics.