CO2 refrigerant is expected to be use for air conditioners due to its environmental property of GWP = 1, non-flammability and low toxicity and has already been applied to hot water supply system, refrigerators and freezers. However, since cycle COP of CO2 is poorer than that of HFC in applying to air conditioner, it cannot be expected to improve the performance significantly by simply improving the efficiency of a conventional single-stage cycle and components consisting of the cycle.  Therefore, it is important to improve the cycle efficiency, especially in the performance of cooling which is a weak point of CO2. Challenges also lie in reducing weight of components such as compressor and heat exchangers resulted in weight gain to resist high pressure and to achieve high efficiency in the system. In this study, a prototype of CO2 refrigerant VRF air conditioning system is constructed and evaluates the performance. The system is adopting the four-stage compression cycle applying a new type compressor and heat exchangers and so on aimed for high efficiency cycle and reducing weight As a result of the evaluation, it is confirmed that CO2 cycle performance can be improved in similar extent to HFC refrigerant cycle. On the other hand, this high efficiency cycle consists of a lot of components and becomes more complex than HFC refrigerant cycle; therefore the size and weight of outdoor unit increases to house these components. Further technical development will be necessary to overcome these weaknesses
