An experimental study of the decomposition of methane and carbon dioxide using capacitive RF discharge was investigated over a moderate range of pressures (5-60 Torr). The decomposition of methane and carbon dioxide molecules is caused by direct electron collision via excitation of the unstable electronic state. Mechanisms of dissociation of methane and carbon dioxide by electron impact were proposed and the conversions of each gas were derived from the proposed mechanisms. As a result, the conversion of each gas only depended on the specific energy of the molecules. The energy costs for the decomposition of methane and carbon dioxide could be determined through the experimental data. The major gaseous products of the decomposition of methane were hydrogen and C-2 and/or C-3 compounds. In the case of methane, some different reaction mechanisms were observed on the range of input power, and a small amount of film deposition inside the reactor by polymerization was observed at high input power. Carbon monoxide was mainly produced from the decomposition of carbon dioxide.