Synthetic diamond particles were deposited on a Si(100) substrate using a hot-filament chemical-vapour-deposition method in order to study the effect of the reaction pressure on the nucleation behaviour. The reaction pressure was controlled, as an experimental variable, from 2 to 50 torr under the following conditions : a filament temperature of 2200-degrees-C, a substrate temperature of 850-degrees-C, a total flow rate of 200 s.c.c.m. and a methane concentration of 0.8 vol %. Diamond deposits on the Si wafer were characterized by micro- Raman spectroscopy, scanning electron microscopy (SEM) and optical microscopy. The maximum nucleation density of diamond particles on the unscratched Si substrate is shown at the reaction pressure of 5 torr. These phenomena can be explained by the competition effect between beta-SiC formation, which increases the diamond nucleation density, and atomic-hydrogen etching which decreases the nucleation sites. A new fabrication method for a high-quality diamond film without any surface pretreatments is introduced using a combination process between diamond nucleation at low pressure (5 torr) and growth at high pressure (30 torr).