In order to reduce the global warming potential resulting from SF6 widely used as an insulating and arc quenching medium, the substitutes need to be found. This paper focuses on different cold C3F8 mixtures (at room temperature) as an insulating gas and hot C3F8 gas (at temperatures of 300–3500 K) as an arc quenching medium, which seem to be a good replacement of SF6. Firstly, the dielectric properties, including the reduced ionization coefficient α/N, reduced electron attachment coefficient η/N and reduced critical electric field strength (E/N)cr, of the cold C3F8-CF4, C3F8-CO2, C3F8-N2, C3F8-O2 and C3F8-Ar mixtures are calculated numerically using the two-term approximation of the Boltzmann equation. The dependence of such dielectric properties on the buffer gas proportion is investigated. Among the various C3F8 mixtures, the C3F8-N2 mixture has the lowest α/N and the C3F8-CF4 mixture has the largest η/N, and moreover, the C3F8-N2 mixture is the best insulator in terms of breakdown strength because it has the largest (E/N)cr. Secondly, the (E/N)cr of hot C3F8 at temperatures up to 3500 K and various pressures is determined and compared with that of hot SF6 gas. It is found that the hot C3F8 gas has much poorer dielectric performance than hot SF6 because the (E/N)cr of C3F8 decreases significantly above room temperature.