Decomposition of CO2 in a fast electric discharge flow of CO2/He or CO2/Ar mixtures was studied by observing UV and visible emission spectra. The decomposition efficiency of CO2 in CO2/Ar mixtures was higher than that in CO2/He mixtures. More than 90% of CO2 in CO2/Ar mixtures was selectively decomposed into CO and O at a microwave power of 100-200 W, and CO2 and Ar flow rates of 25 and 1000 seem, respectively, at a total pressure of 0.5 Torr.