Owing to its low global warming potential (GWP)and high dielectric performance,fluorocarbon gas c- C_4F_8 is a potential substitute for SF_6. Gas insulated switchgear filled with c-C_4F_8/N_2 gas mixture will become a new type of environmentally-friendly equipment. With a combination of heat transfer analysis and experimental measurement,the cooling performances of fluorocarbon gas and its mixture with N_2 at pressures of 0.1 MPa and 0.25 MPa has been studied using a gas-insulated transmission line(GIL)and the results are compared with those of N_2 and SF_6 gas. Results show that the equivalent heat transfer coefficient of the same gas increases with pressure while for different gases we have SF_6c-C_4F_8>c-C_4F_8/N_2>N_2. However, the effect of heat transfer coefficient is nonsignificant to the overall rate of heat loss. The cooling performance of the above mentioned gases is similar.