To develop environmentally friendly SF6-free gas circuit breakers, it is necessary to study characteristics of the alternative mediums. In this work, CO2 which has lower global warming potential, together with its gas mixtures with SF6, is investigated. Based on a 126 kV puffer-type circuit breaker, a series of experiments at different concentrations of SF6 is carried out. Analysis is performed on arc voltage, arc conductance and puffer pressure which are vital factors to estimate the arc-quenching property of the gaseous working mediums. It is found that with the increase of concentration of SF6, arc voltage, extinction voltage and arc resistance increase either. And adding SF6 gas to CO2 gas leads gas blow to be more intense and therefore enhance arc-quenching capability of gases.