Nanocrystalline diamond films have been prepared by using a direct current arc plasma jet system. Hydrogen flow rate of the chemical vapor deposition process was decreased to induce the transition from micro- to nanocrystalline diamond films. The deposition rate was found to decrease from 20 mum/h for the microcrystalline to 8 mum/h for the nanocrystalline diamond films. It is believed that this proves that the growth of diamond films is somehow facilitated by atomic hydrogen abstractions. On the other hand, the growth of nano diamond films was found to be a quenched process, with a result of formation of colony-featured microstructures. For the nanocrystalline diamond films, the problem of surface roughness remains, not because of faceting of individual diamond grains, as in the case of microcrystalline diamond films, but because of suppressed diffusion of depositing atoms. It is shown that there exists a close relationship between surface roughness of the deposited nanocrystalline diamond films and the substrates, and an improvement in flatness of substrate surfaces could lead to a decrease in roughness of the deposited nanocrystalline diamond films. (C) 2003 Elsevier Science B.V. All rights reserved.