The experiments were carried out on a verti-COIL, with the chlorine flowrate of 155 mmol/s, which was designed for N2. The effect on the output power was studied, with different import positions, different dilution ratios and different buffer gases, N2, Ar, and CO2. The buffer gas introduced from singlet oxygen generator(SOG) was positive to the output power and it increased with the dilution ratio within a certain range(0~1.0). The output power differed from each other when different buffer gases were used. The calculation indicated that Ar gas showed better performance in improving the small signal gain coefficient and lowering the temperature of gas.