Hard carbon films were produced by the rf glow discharge plasma decomposition of hydrocarbon gases diluted with H2gas at relatively low temperature and low pressure. Optical emission spectra were measured during the deposition of the carbon films, and the correlation between the carbon film’s properties and the plasma’s condition was examined. From these results, H radicals were considered to contribute to the formation of hard carbon films. These were examined by transmission electron microscopy, and were found to be predominantly amorphous. The existence of diamond grains was confirmed in the amorphous phase. A relatively large diamond grain in the order of 10 &mgr;m was obtained under the conditions of a gas mixture ratio of 70% and a gas pressure of 5×10−3Torr.