||Air-source heat pumps are widely used in residential heating system. However, the decrease in the capacity of the heat pump is unavoidable when operating at very low and high ambient temperatures. The vapor injection technique is considered a promising technology to overcome this problem. Recent research on vapor injection cycles have mainly adopted a scroll compressor with an internal heat exchanger at severe operation conditions. This study measured the COP and EER of a gas injection heat pump using a flash tank with an inverter-driven rotary compressor at severe operation conditions. Compared to non-injection heat pumps,the heating capacity and COP of the has injection heat pump improved up to 15% and 2.9%, respectively, at outdoor temperatures of -10 °C to 7°C. The cooling capacity of the gas injection heat pump was 11% higher than the non-injection heat pump at an outdoor temperature of 35°C. At the same time, the EER of the gas injection heat pump was similar to that of the non-injection heat-pump.