The only way to guarantee that a refrigerant is not harmful to the environment is to use a natural substance. CO2 is chemically inert, harmless and nonflammable. The good heat transfer properties and high volumetric cooling capacity also enhance the use of CO2. Ground source energy and solar energy are 100 % renewable energy sources that are available everywhere and that work well with heat pump system. This study describes a solar assisted ground source CO2 heat pump to provide space heating. The effects of the gas cooler outlet temperature and high side pressure on the system COP were studied also. The gas cooler outlet temperature strongly influences the cycle performance with the COP increasing with decreasing outlet temperature. Experimental tests of the ground source CO2 heat pump show that the COPH of the CO2 heat pump in space heating mode exceeds 3.0. The solar assisted ground source CO2 heat pump system had COPHs greater than 3.5 for the evaporation temperature was increased by 1.5 ℃ by 9.6 m² of solar collectors.