In this paper, the nucleation of diamond films was studied with the varied bias value and mass current density by adjusting the gas flow rate through the chamber. Results showed that the two parameters above greatly influenced the nucleation density and the morphology of diamond films. Both bias value and mass current density had an optimum value in which the nucleation density reached the maximum. The nucleation of diamond films by hot filament chemical vapor deposition (HFCVD) is a competitive process of deposition and etches (sputtered) due to particle impact on the substrate. The magnitude of the particle energy in gas plasma and mass current density of gas controlled the competitive process and determined whose process was primary. The deposition process was gradually strengthened and the nucleation density increased due to the increment of the energy and quantity of the adatoms and reached a maximum at optimization with the increment of the two parameters mentioned above. The deposition process is suppressed by strengthened particle impact on substrate and the etch (sputtered) process was primary with the further increment of the two parameters above.