Insulating gas SF_6 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 SF_6 due to its high global warming potential (GWP). Consequently, we studied c-C_4F_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_4F_8/N_2 mixture gas with different gas ratio k were measured in different gas pressures (0.1~0.65 MPa) and gap distance (1~7 mm) and then compared with that of pure SF_6. The results show that the c-C_4F_8/N_2 mixture gas presents a synergistic effect. The breakdown strength of mixture gas with 20% c-C_4F_8 is about 0.6 times that of pure SF_6 and the flashover strength is about 0.7 times. The two-hour withstand voltage is 5%~10% lower than the flashover voltage. There is a possibility that the c-C_4F_8/N_2 mixed gas can be used for substituting SF_6, with increasing the gas pressure or insulation dimension. Because the carbon granule is one components of decomposition products for discharged c-C_4F_8 gas, the application of the c-C_4F_8/N_2 mixture gas in frequent discharge devices should be avoided.