Diamond films have been deposited by d.c. discharge assisted hot filament chemical vapor deposition. The diamond nucleation density was significantly enhanced more than five orders of magnitude by this d.c. discharge assisted process. The effects of deposition parameters including deposition time, temperature, and discharge current on the diamond growth and gas phase composition were studied by Raman scattering, scanning electron microscopy, and optical emission spectroscopy. The mechanisms of the discharge enhanced nucleation of diamond are discussed.