Nanocrystalline diamond film was deposited by microwave CVD in the ASTeX type reactor on a mirror polished (111) oriented n-doped silicon substrate. The deposition mixture consisted of 9 % of methane in hydrogen. The applied microwave power (2.45 GHz) and pressure were 850 W and 7.5 kPa, respectively. The substrate temperature was 1 090 K. The diamond nucleation process was enhanced by rf induced dc self {bias of -125 V. The film exhibited very low roughness (rms of heights 9.1 nm). Its hardness and elastic modules were 70 and 375 GPa, respectively. The optical constants were determined by combination of spectroscopic ellipsometry and reflectometry employing the Rayleigh-Rice theory for the roughness and the dispersion model of optical constants based on the parameterization of densities of states. The deposition rate was 57 nm/min including the 5 min nucleation step.