The direct conversion of methane (CH4) into methanol (CH3OH) and HCHO in an Argon (Ar) 50W radio-frequency plasma system was applied and the effects of various feed compositions, CH4/O2 ratio, and plasma discharge areas were compared. Additionally, the effects of various methane to oxygen ratios and plasma discharge areas were studied. It was found that in an Ar stream, the CH3OH conversion ratio in the CH4/O2 plasma system was higher than that in CH4/CO, CO/H2 and CH4/H2/O2 plasma systems. The conversion of CH4 reached 19.1% at CH4/O2 = 40/60; the yield of CH3OH was 1.12% and 16.0% CO, the major product, was produced. A larger plasma discharge area, resulting in a longer residence time, corresponded to higher CH4 conversion, but a lower CH3OH conversion ratio, because of further decomposition into CO and CO2. Interestingly, no carbon deposition was observed in the RF plasma system, and the carbon balance was between 0.94 and 1.19.