SF6 has been widely used in electrical industry because of its excellent electrical capabilities. However, due to its extremely high Global Warming Potential (GWP), researchers have done lots of investigations on SF6 alternative gases during the last few decades. Unfortunately, all the substitutions have their own drawbacks and limitations and can't be applied in practice. In this paper, the critical electric field strength under AC voltage of new environment-friendly gas mixtures, C5F10O/CO2, was studied as the main object. The partial pressure of C5F10O was set to be 20 kPa and 40 kPa, and the total pressure varied from 0.1 MPa to 0.7 MPa. SF6 and CO2 were also studied for comparison. In considering of the liquefaction temperature of the mixtures, the saturated vapor pressure characteristic of C5F10O was tested. The saturated vapor pressure shows that C5F10O has high enough vapor pressure to make the use of its gas mixture as a possible substitution of SF6. Finally, the possibility of this gas mixture to replace SF6 was discussed. The results show that the insulating performance of the gas mixtures can be effectively improved by increasing the content of C5F10O.