The objective of this study is to optimize the performance of a gas injection CO2 heat pump according to the operating parameters in extreme heating and cooling conditions. The performances of CO2 heat pumps using a flash tank gas injection (FTGI) in the heating mode and FTGI with the suction line heat exchanger (FTSX) in the cooling mode are measured with the variations of the injection ratio, compressor frequency, and evaporating pressure in the extreme outdoor conditions. At the compressor frequency of 45 Hz, the cooling COP of the gas injection CO2 heat pump with the optimum injection ratio is 6.8% higher than that of the non-injection CO2 heat pump at the indoor/outdoor temperatures of 17/43 degrees C. Moreover, the heating COP of the gas injection CO2 heat pump is 7.1% higher than that of the non-injection system at the indoor/outdoor temperatures of 20/-15 degrees C. (C) 2018 Elsevier Ltd. All rights reserved.