Nanocrystalline diamond films were deposited on Co-cemented carbide substrates using CH4/H2/Ar gas mixture by hot filament chemical vapor deposition (HFCVD) technique. The evidence of nanocrystallinity, smoothness and purity was obtained by characterizing the sample with scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), high-resolution transmission electron microscopy (HR-TEM) and selected-area electron diffraction (SAED). A new process was used to deposit composite diamond films by a two-step chemical vapor deposition procedure including first the deposition of the rough polycrystalline diamond and then the smooth fine-grained nanocrystalline diamond. The results show that the film consists of nanocrystalline diamond grains with sizes range from 20 to 80 nm. 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<100nm by AFM. Smooth nanocrystalline diamond layers can be deposited on conventional microcrystalline diamond layers using a two-step chemical vapor deposition by regulating the deposition parameters. These composite diamond films with the multiplayer (nanocrystalline/microcrystalline) structure have low surface roughness and high adhesive strength on WC-Co substrates. The diamond-coated tools and drawing dies with these composite coatings display excellent performances in the practical application.