This study presents a CO2 heat pump cycle with a vortex tube in which a vortex tube is applied to replace the throttle valve used in the conventional CO2 heat pump cycle. The COPh and total heating capacity of this cycle are theoretically investigated and compared with that of conventional CO2 heat pump cycle for the ranges of evaporating temperature (−15 to 3 ℃ ) and gas-cooler pressure (8 to 9.8 MPa). The COPh of this cycle increases with the increasing of nozzle efficiency, the evaporating temperature and the heating effect of vortex tube; moreover, the COPh of this cycle increases with the decreasing of the temperature of gas cooler outlet and the cold mass fraction. Under the same operating conditions, the COPh of this cycle can be improved by up to 3.9 ~ 16.8% over the conventional CO2 heat pump cycle.