Diamond nucleation on very smooth (100) silicon substrates coated with thin films of a colloidal graphite suspension was investigated with a microwave‐plasma‐enhanced chemical‐vapor‐deposition system. Nucleation densities of the order of 106cm−2were obtained by coating the substrates with carbon films of thicknesses less than 1 &mgr;m. However, very low nucleation densities were obtained with carbon film thicknesses greater than 1 &mgr;m. The effect of the carbon film thickness on diamond nucleation was examined by measuring the etching rate of carbon films exposed to a hydrogen plasma and was further interpreted on the basis of scanning electron microscopy and Raman spectroscopy results. Etching of the original carbon may lead to the formation of a thin residual carbon film when the initial film thickness is less than a critical value. Results demonstrated that the high nucleation densities of good quality cubo‐octahedral diamond crystals obtained with relatively thin carbon films were primarily due to the formation of a porous ultrathin residual carbon film. The critical initial film thickness was a function of the plasma etching and deposition rates of carbon which, in turn, affected the effective local carbon concentration. Thick carbon films yielded insignificant nucleation densities and poor quality diamond because of the high local carbon content resulting from the partial etching of carbon and the increased carbon concentration in the plasma. The local carbon concentration and the residual carbon film are the proposed principal factors for the obtained high diamond nucleation densities on unscratched silicon substrates.