Abstract: CF₃I is a potential SF₆ alternative gas. In order to study the insulation properties and synergistic effects of CF₃I/N₂ and CF₃I/CO₂ gas mixtures, two-term approximate Boltzmann equations were used to obtain the ionization coefficient α, attachment coefficient η and the critical equivalent electrical field strength (E/N)_cr. The results show that the (E/N)cr of CF₃I gas at 300 K is 1.2 times that of SF₆ gas, and CF₃I/N₂ and CF₃I/CO₂ gas mixtures both have synergistic effect occurred. The synergistic effect coefficient of CF_3I/CO₂ gas mixture was higher than that of CF₃I/N₂ gas mixture. But the (E/N)_cr of CF₃I/N₂ is higher than that of CF₃I/CO₂ under the same conditions. When the content of CF₃I exceeds 20%, the (E/N)_cr of CF₃I/N₂ and CF₃I/CO₂ gas mixture increase linearly with the increasing of CF₃I gas content. The breakdown voltage of CF₃I/N₂ gas mixture is also higher than that of CF₃I/CO₂ gas mixture in slightly non-uniform electrical field under power frequency voltage, but the synergistic effect coefficients of the two gas mixtures are basically the same.