The electron drift velocity, electron energy distribution function and density-normalized effective ionization coefficient are calculated for SF6-CHF3 and SF6-CF4 mixtures. Two-term, spherical harmonic expansion approximation is utilized to calculate the swarm parameters in steady-state Townsend form Boltzmann analysis. Results show that the drift velocity varies feebly with SF6 content. On the other hand, a strong dependence is observed in density-normalized effective ionization coefficient with SF6 content, the gas mixture becomes more electronegative as the share of SF6 in gas mixtures increases. The calculated value of SF6-CHF3 and SF6-CF4 mixtures is lower than measured value of SF6-N2. The calculated density-normalized effective ionization coefficients obtained in the paper are compared with measured data from pulsed Townsend experiments. The predicted results in the numerical simulation are in agreement with the experimental results.