AC breakdown strengths of CO2, N2, SF6 and air in non-uniform field were experimentally studied. The experimental results have shown that the breakdown voltages of SF6 in the practical range of pressure (100-200 kPa) are always higher than those of other gases. Although at short gaps, the breakdown strength of SF6 is superior at the pressure range from 100-500 kPa, at 25 mm electrode gap spacing and 300 kPa the breakdown voltage of air is %7.8 higher than that of SF6. At above pressures of 400 kPa and 15 mm electrode gap spacing, there exists a critical field where the breakdown voltage of CO2 has a maximum value.