In order to save the computing time of Monte Carlo simulation, traditional null collision method is improved in which only one cross section needs to be evaluated at each collision speed. By this method, the electron swarm parameters such as effective ionization coefficient, $\bar{\alpha}$, drift velocity and diffusion coefficient of SF6 and CF4 gas mixtures are calculated over the E/N range from 150 Td to 500 Td. A set of electron collision cross-sections that gives transport parameters in agreement with measurements for the pure gases is used. The simulation results are then compared with our experiment data, and show excellent agreements.