In this study, experiments were performed to measure the inception and breakdown voltages of SF6/CO2 mixed gases with varying mixing ratios at two different pressures (1 and 2 bar). Using the preliminary data obtained from the experiments, a 3D particle modelwas used to investigate positive streamer initiation in SF6/CO2 with field ionization. Field ionization using Zener's model was assumed to be similar to the electron detachment in air. The experimental results showed that at P = 1 bar, the discharge inception voltage increased for CO2 concentrations lower than 50% and decreased for CO2 concentrations higher than 50%. The inception voltage of 80%CO2/20%SF6 was close to the inception voltage of pure SF6. A similar trend of the inception and breakdown voltages was observed at P = 2 bar. The simulation results show the evolution of the thin filaments of the streamer into three distinct stages. The streamer morphology was similar irrespective of the type and concentration of the mixed gas. Other parameters such as the electric field, electron density, and streamer length increased with increasing CO2 concentration in SF6 at both pressures. Zener's field ionization could be considered a primary ionization mechanism for SF6/CO2 in the case of very high non-uniform electric fields; however, finding the experimental validation of Zener's field ionization for electronegative gases is still a challenging task.