Nano-crystalline diamond films (NDF) possess the same excellent properties as that of poly-crystalline diamond films; moreover, NDFs are composed of nano-diamond grains, so it is easy to grow smooth, continuous, uniform ultra-thin NDFs films. Without difficult and expensive polishing, NDFs can be directly used in many fields as mechanical and optical protective coatings, wear resistant coatings of mechanical matings, components in nano-devices, excellent electrochemical electrodes and field emission devices, ect. Now, the investigation of the synthesis technique, growth mechanism and applications of NDFs is a hot-point in CVD diamond films research.In this work, the synthesis technique by conventional HFCVD method has been developed to cheaply grow NDFs at low temperature (< 600 ℃) and low pressure (~ 7 Torr) without adding of inert gases such as Ar and bias voltages between the filament and the substrate. The HFCVD method has many outstanding advantages, such as, it is easy to grow large-area uniform films or many samples at the same time, its equipment is simple and operating cost is low. The applications of NDFs in industry will be promoted by this NDF growth technique combined with the large-area HFCVD diamond film synthesis technique developed by our group.Our NDF synthesis technique includes the following steps: â‘  the sufficient ultra-sonic pretreatment of the substrate; â‘¡ HFCVD nucleation at the optimal conditions; â‘¢ atomic hydrogen etching; â‘£ HFCVD growth at the optimal conditions; ⑤ annealing (cooling down to room temperature).The results of the ultra-sonic pretreatment show that: the grain size of the diamond powder plays an important role in promoting diamond nucleation. The larger the diamond powder grains (from 0.5 to 40 μm), the higher the nucleation density (ND). Among the four liquids, acetone is the best. At optimal HFCVD nucleation conditions, the ND higher than 10~(11) cm~(-2) on Si surface can be obtained by 1 hour ultra-sonic pretreatment using 40 μm diamond powder, 250 mg in 20 ml acetone. The diamond nuclei size and distribution on the substrate are uniform. The ultra-sonic pretreatment technique developed in this work is simple, efficient and economic. During the ultra-sonic pretreatment, the growth surface of the substrate is only little damaged and the other surface keeps intact, therefore the transmittance of optical substrates is almost not affected.The influence of CH_4 concentration on the ND is the most significant in all HFCVD parameters. The higher the CH_4 concentration (0.5 to 2.5 %), the higher the ND. At 1 % CH4 concentration, the ND reaches the highest at substrate temperatureof 800-830℃. At lower substrate temperature (450 to 600℃), in order to obtain high ND, the CH_4 concentration must be higher than 2 %. The ND increases further with the decrease of pressure from normal 15-30 Torr to 7 Torr. Under optimal conditions, ND higher than 10~(10)cm~(-2) on silica and optical glass substrates can be obtained. The ND as high as 1.5×10~(11) cm~(-2) has been reached on Si substrate at ~ 550℃ and ~ 7 Torr pressure, this is the highest ND at lower temperature by conventional HFCVD method reported in literatures.CH_4 concentration also plays an important role in the micro-structure and optical transmittance of the NDF. When CH_4 concentration is lower than 0.5%, the obtained diamond films are composed of 150-200 run grains with apparent diamond facets. Grown at CH_4 concentration higher than 1.5 %, the grain sizes decreases to less than 100 nm without diamond facets. The mean transmittance reaches the highest after nucleation for 8 min at ~ 1 % CH_4 concentration. The mean transmittance almost has no change for the samples etched by H° for different time. The mean transmittance decreases with the increase of the CH_4 concentration at the growth step. The thickness of the non-crack film, adhesion, growth rate and the optical absorption coefficient of the NDF are dependent on the substrate temperature. The Vis-NIR transmittance of the NDF with the thickness of 500 nm and without cracks grown on the silica substrate at 650℃ is higher than 60 %, and is sufficiently high for most practical optical applications.The computer dissolved Raman spectra show the characteristic peaks of the NDF located at 1140, 1200 and 1480 cm~(-1) exist in all NDFs, the main non-diamond Raman peaks are the characteristic peaks of graphite near 1350 and 1580 cm~(-1). The fitting spectra match very well with the corresponding measured spectra.By analysis of Raman spectra, FE-SEM, AFM, HR-TEM, SAED, XRD and optical transmittance, it is proved that the films synthesized by our HFCVD technique are smooth, continuous and uniform NDFs, with grain sizes in a range of 10-50 nm and less than 500 nm thicknesses.High ND and lower growth temperature (<600℃) make the synthesis of the excellent NDF on optical glass possible. The low substrate temperature, the ultra-thinness of the NDFs, the optimal HFCVD parameters and the annealing process effectively reduce the total strain in the NDFs and the adhesion between the NDFs and the substrates is improved. The good adhesion of the NDFs on silica or optical glass substrates is beneficial for the applications of the NDFs as protective coatings on optical devices and mechanical matings.