Diamond nucleation on (111)-oriented monocrystalline silicon wafer was investigated by hot filament chemical vapor deposition (HFCVD). The variation of nucleation density with time was determined. For a lower gas flow rate, the nucleation density-time curve comprises two parts, which represent the nucleation at surface defects and smoothly intact surface sites. For a higher gas flow rate, the distinction between the two parts in the curve tends to vanish. Diamond nucleation was enhanced by increasing the gas flow rate.