Nanocrystalline diamond films (NDFs) were deposited on mirror-polished silicon (100) substrates using a graphite-grid assisted hot filament chemical vapor deposition (HFCVD) technique. The evidence of nanocrystallinity, smoothness and purity was obtained by characterizing the sample with various advanced analyses. A graphite-grid was used as the DC electrode, which was pre-coated by diamond films and could emit electron when a negative bias was applied to the under electrode. The results show that the film consists of nanocrystalline diamond grains with sizes of about 7-15nm. The Raman spectroscopy, XRD pattern, HR-TEM image and SAED pattern of the films indicate the presence of nanocrystalline diamond. Surface roughness is measured as Ra<20nm by the profilometry scans. A large quantity of electrons emission from the graphite-grid and positive ions bombardment to the graphite-grid results in an enormous enhancement of the generation of diamond nuclei. Density of a diamond nuclei as high as 1010~1011/cm2 can be attained with this method. NDFs can be deposited on mirror-polished Si substrate surfaces without damaging surface pretreatment for nucleation enhancement. These ultra-smooth films will display excellent performances, which make them the best candidates for semiconductor and MEMS applications.