Biogas reforming with added O-2 through a spark-shade plasma was conducted under O-2/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 O-2 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. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.