We have investigated the ultrananocrystalline diamond (UNCD) deposition by direct-current plasma assisted chemical vapor deposition on 4 in. Si wafer using CH4H2 as well as CH4Ar gas chemistry containing additive nitrogen. CH4/N2/H2 (5%/0.5%/94.5%) and CH4/N2/H2/Ar (0.5%/5%/6%/88.5%) gas mixtures were compared as the precursor gas. Molybdenum and tungsten were compared as cathode material. Discharge voltage and current were 480 V/45 A and 320 V/60 A, for respective gas chemistry. Chamber pressure and substrate temperature were 110–150 Torr and 750–850 °C, respectively. The film was characterized by near edge x-ray absorption fine structure spectroscopy, x-ray diffraction, high-resolution transmission electron microscope, electron energy loss spectroscopy, and high-resolution scanning electron microscope. We have demonstrated that (1) elimination of the positive column, by adopting very small interelectrode distance, gave some important and beneficial effects; (2) the plasma stability and impurity incorporation was sensitive to the cathode material and the precursor gas; (3) using the conventional CH4/H2 precursor gas and tungsten cathode, the mirror-smooth 4 in. UNCD film of excellent phase-purity and grain size below 10 nm could be deposited even in the absence of the positive column. The high electric field in the unusually narrow interelectrode space and the consequent high electron kinetic energy, in conjunction with the unusually high electron current thereof, directed to the s- - ubstrate, i.e., the anode, was proposed to be the source of the grain refinement to achieve UNCD at such high chamber pressure around 110–150 Torr, in the absence of the usual ion bombardment assistance.