To find a friendly alternative gas with low global warming potential (GWP) for SF_6,we investigated insulating properties of c-C_4F_8/N_2 and c-C_4F_8/CO_2 mixtures.Meanwhile, experiments in a sphere-plate electrode were done in different gap distance, pressures,and gas ratio of c-C_4F_8/N_2 and c-C_4F_8/CO_2 mixture gas under frequency AC voltage, compared with SF_6, c-C_4F_8,N_2 and CO_2 in the same conditions. And the decomposition products were detected after breakdown. Experimental results show that the breakdown voltage of mixtures increases as the gas pressure/gap length/ratio of c-C_4F_8 increases; there is a synergistic effect between c-C_4F_8 and N_2/CO_2; In slightly nonuniform field, insulation performances of 30% c-C_4F_8/N_2 and c-C_4F_8/CO_2 mixed gas are equal to 75% and 79% of pure SF_6 in 0.1 MPa; the synergistic effect coefficients of c-C_4F_8/N_2 and c-C_4F_8/CO_2 in 5%~30% mixing ratio are between 0.20~0.79 and 0.15 ~0.65,respectively. From the insulating properties,liquefaction temperature standpoint and environment, c-C_4F_8/N_2 and c-C_4F_8/CO_2 gas mixture may be used as an alternative for SF_6 gas in electrical equipment.