At present work AC breakdown strengths of N-2, CO2 SF6 and the mixtures of SF6+N-2 and SF6+CO2, containing 20, 10, 1% of SF6 in non uniform field were experimentally studied. The rod-plane of 5, 15 and 25 mm gap spacing was used as an electrode system. The relative gas pressure was varied within the range of 100-500 kPa. The experimental results have shown that addition of SF6 to CO2 and N-2 can significantly increase the AC breakdown voltage with respect to CO2 and N-2 Especially within the pressure range of 150-300 kPa, the breakdown voltages of same of the SF6+ CO2 mixtures are higher than that of pure SF6. For example, the breakdown voltages of 20, 10% SF6 mixtures far 25 mm gap spacing are about 2.4 and 2.8 times those of CO2 and N-2 respectively at a pressure of 200 kPa. In the case at 25 mm gap spacing, mixtures with 1% of SF6 content have breakdown levels same as the corresponding values in pure CO2 and N-2 gas pressure in excess of 400 kPa and 350 kPa respectively.