The deposition of diamondlike carbon (DLC) film and the measurements of ionic species by means of mass spectrometry were carried out in a CH4/N-2 RF (13.56 MHz) plasma at 0.1 Torr. The film deposition rate greatly depended on both CH4/N-2 composition ratio and RF power input. It was decreased monotonically as CH4 content decreased in the plasma and then rapidly diminished to negligible amounts at a critical CH4 content, which became large for higher RF power. The rare increased with increasing RF power, reaching a maximum value in 40% CH4 plasma. The predominant ionic products in CH4/N-2 plasma were NH4+ and CH4N+ ions, which were produced by reactions of hydrocarbon ions, such as CH3+, CH2+, CH5+, and C2H5+ with NH3 molecules in the plasma. It was speculated that the production of NH4+ ion induced the decrease of C2H5+ ion density in the plasma, which caused a reduction in higher hydrocarbon ions densities and, accordingly, in film deposition rate. The N-2(+) ion sputtering also plays a major role in a reduction of film deposition rate for relatively large RF powers. The incorporation of nitrogen atoms into the bonding network of the DLC film deposited was greatly suppressed at present gas pressure conditions.