The thermodynamic performances of solar assisted CO2 vapor compression heat pump cycles with an ejector are analyzed and compared with that of conventional CO2 heat pump cycle (named as A cycle). Under the given working conditions, the COP of solar assisted CO2 heat pump cycle with an ejector after compressor cycle (named as C cycle) is much higher than that of CO2 heat pump cycle with an ejector before compressor cycle (named as B cycle) and that of the A cycle because of the lower compressor power of the C cycle. The compressor volume displacement of the C cycle demanded for providing the same heat capacity of gas-cooler is the lowest among the three cycles. So the compressor size of the C cycle is very small and the cost of the C cycle is very less. In the area having rich solar energy resource, it is significant to employ the C cycle for providing space heating by optimizing design ejector and selecting compressor, thus, the performances for this C cycle can be improved greatly. In the area not having rich solar energy resource, the B cycle can be used for providing space heating; the performances can also be improved accordingly.