Insulating gas SF6 has been used in gas insulated line（GIL） due to its excellent electrical capabilities. However

it is urgently required to find a gas to substitute SF6 due to its high global warming potential（GWP）. Consequently

we studied c-C

4

F

8

/N

2

mixture gas with gas ratio lower than 20% as the main object. The DC breakdown voltage and flashover voltage of c-C

4

F

8

/N

2

mixture gas with different gas ratio k were measured in different gas pressur

es（0.1

0

.65 MPa） and gap distance（1

7

mm） and then compared with that of pure SF6. The results show that the c-C

4

F

8

/N

2

mixture gas presents a synergistic effect. The breakdown strength of mixture gas with 20% c-C

4

F

8

is about 0.6 times that of pure SF6 and the flashover strength is about 0.7 times. The two-hour withstand voltage is 5%

1

0% lower than the flashover voltage. There is a possibility that the c-C

4

F

8

/N

2

mixed gas can be used for substituting SF6

with increasing the gas pressure or insulation dimension. Because the carbon granule is one components of decomposition products for discharged c-C

4

F

8

gas

the application of the c-C

4

F

8

/N

2

mixture gas in frequent discharge devices should be avoided.