Diamond was deposited on smooth Si3N4-coated Si substrates using hot-filament chemical vapor deposition (CVD). Prior to diamond deposition, the substrates were pretreated under high temperature (700–900 °C) by decomposed pure hydrogen. The experimental results show that these pregrowth processes at a substrate temperature of 850 °C enhanced the diamond nucleation on Si3N4. This indicated that decomposed hydrogen acted on the substrate to produce active centers (<100 nm), which promoted diamond nucleation. It has also been suggested that Si3N4 coating can inhibit carbon transport into the substrate, and it is beneficial to the diamond nucleation.