Biogas reforming with added O2 through a spark-shade plasma was conducted under O2/CH4 ratio of 0.60 and a specific energy input (SEI) of 69 kJ/mol with energy efficiency of around 75%. Effect of CO2/CH4 ratio ranging from 0.17 to 1.00 on the biogas reforming was investigated, and the discharge characteristics of the plasma showed nearly same. With the increase in CO2/CH4 ratio, the conversions of O2 and CH4 decreased, however, that of CO2 apparently increased followed by a little reduction. Also the effect of CO2/CH4 ratio on C-based and H-based selectivities, C-balance and H-balance, and dry-basis concentration and energy cost of syngas were examined. The effective factor of syngas production per CH4 conversion increased with CO2/CH4 ratio, indicating that CH4 conversion to syngas was promoted with the increase in CO2/CH4 ratio. The combination coefficients of independent reactions confirmed that partial oxidation of methane mostly contributed to CH4 conversion and reverse water gas shift solely did CO2 conversion.