Ultrananocrystalline diamond (UNCD) films, which possess very smooth surface, were synthesized using CH4/Ar plasma. When the nucleation process was carried out under methane and hydrogen (CH4/H2) plasma with negative DC bias voltage, no pretreatment on substrate was required prior to the formation of diamond nuclei. The average grain size of BEN induced diamond nuclei is about 20∼30 nm, with the nucleation site density more than 1011 sites/cm2. The growth rate of UNCD is markedly enhanced due to the application of BEN induced nuclei. Moreover, the growth rate of UNCD films was more significantly affected by the substrate temperature, but was less influenced by the microwave power. All of these UNCD films showed similar morphology, i.e., with grain size less than 10 nm and surface roughness around 10 nm. They also possess similar Raman spectra, i.e., similar crystallinity. However, the deposition rate can be increased from ∼0.2 to 1.0 μm/h when substrate temperature increased from 400 to 600 °C.