The critical reduced electric field strength (E/N)cr is an important parameter for the prediction of dielectric breakdown performances of SF6 and its mixtures. This paper aims to analytically investigate and compare the dielectric breakdown properties of SF6-N2 and SF6-CF4 mixtures at different concentrations of SF6, 0.4 MPa and gas temperatures up to 3000 K. First, the equilibrium compositions of those gas mixtures are calculated, by minimizing the Gibbs free energy under the assumptions of local thermodynamic and chemical equilibrium. Then the (E/N)cr of those hot gases, defined as the value for which the balance is reached between the total ionization reaction rate coefficient and the total attachment reaction rate coefficient, are determined by Boltzmann equation analysis. It is found that for T > 2000 K, both of those two mixtures have superior dielectric breakdown properties to pure SF6, and (E/N)cr increases with the concentration of SF6 decreasing. Different with the cases in pure SF6 and SF6–N2 mixtures, for T > 2500 K, the (E/N)cr in SF6–CF4 mixtures increase with temperature, as a result of the productions of dissociation of CF4, in particular CF2. In addition, at all the gas temperature range considered, the (E/N)cr in 50% SF6-50% N2 is obviously higher than that in 50% SF6-50% CF4.