Sulphur hexafluoride (SF6) gas and SF6 gas mixtures are widely used in gas-insulated systems (GIS) because they have good dielectric and thermal properties. Among the various gas mixtures investigated so far, SF6-air, SF6-CO2, and SF6-N-2 are most used gas mixtures. Also, these mixtures have much technical superiority in GIS. These are nontoxic and nonflammable gases, and they have less sensitivity to nonuniformities and higher liquefaction temperature at high pressure. GIS, switchgears, circuit breakers, and substations are systems that work with alternating current and have nonuniform electric fields. For this reason, in this study the breakdown mechanisms of 0.125, 0.5, 1, and 20% SF6 gas mixtures with rod-plane configuration under AC voltage are investigated and explained breakdown mechanism.