This paper is aimed at the comparison of breakdown voltages of SF6 and mixtures at different concentrations of Fluoronitriles in CO2. The experiments are conducted in different electrodes geometries under DC and lightning impulse voltages. It is shown that that the 3.7% Fluoronitriles/ 96.3% CO2 mixture constitutes a good compromise and an appropriate gas mixture for high voltage apparatus insulation in point of view of pressure and  low ambient temperature application (-30°C).