The achievement of nanosmooth, ultrathin diamond coatings with nanosized grains is mandatory for the successful utilization of diamond in areas such as microelectromechanical systems, field emission and surface acoustic waves devices. The bias enhanced nucleation technique (BEN) allows to achieve high nucleation density diamond films, where the average distance between diamond nuclei can be as low as 10-20nm. Moreover by diluting the gas precursors (H2 and CH4) into noble gas (Ar, He) during growth, the formation of larger crystals can be inhibited, giving rise to nanocrystalline films without a degradation of the film quality, such as the presence of more graphitic bonds. In this paper we report the growth of ultrathin, smooth, high quality nanodiamond films obtained by combining the two techniques in a HFCVD reactor. A variety of nanocrystalline diamond films with a grain size as low as 10nm and thickness up to 1μm were obtained. The nucleation process and ensuing growth of the film were monitored by SEM observation. Spectroscopic measurements were also performed to study the microstructure and to assess the quality of the deposited material.