The deposition of diamond-like carbon (DLC) film and the measurements of ionic species by means of mass spectrometry were carried out in a CH4/O-2 RF (13.56 MHz) plasma. The film deposition rate greatly decreased with increasing O-2 content up to 10% O-2. The oxygen ion density indicated the remarkable increase with the increase of O-2 content up to 10%. The hydrocarbon ion density is increased considerably by the addition of 5% O-2, and then decreased monotonically for O-2 contents more than 5%. Many oxygenated hydrocarbon ions were detected in the CH4/O-2 plasma. They were classified into three kinds of ions: CnHmO+, CnHmOH+, and CnHmO2H+. When the O-2 content was increased, the higher hydrocarbon CnHm+ (n greater than or equal to 2) ion densities (except C2H4+) decreased more than the principal (CH3+ and CH4+) ion densities, since the generation of higher hydrocarbon ions would be suppressed by the formation of oxygenated hydrocarbons. It was also suggested that the great decrease in the film deposition rate in the CH4/O-2 plasma was mainly associated with considerable decrease in the higher hydrocarbon ion densities. The properties of DLC film obtained in CH4/O-2 plasma did plot indicate considerable change from those obtained in pure CH4 plasma.