Utilization of co2 for transcritical refrigeration cycle this project describes the transcritical refrigeration cycle utilizing co2 as working fluid which is composed with gas cooler, inter cooler, suction line heat exchanger, capillary tube and rolling piston type 2-stage co2 compressor. the adoption of the inter cooler between 1st discharge and 2nd suction reduced the 2nd discharge gas temperature. the adoption of the suction line heat exchanger enabled to have a sufficient superheat of 1st suction. the adoption of capillary tube as an expansion device helped the system simplicity. the better cycle efficiency level was achieved with co2 cycle compared with that of conventional cycle for the application of refrigeration equipment. due to global environmental concerns, the usage of natural working fluid is becoming more interesting theme to be discussed. transcritical co2 cycle is recently considered as one of the most influential refrigerant for its characteristics such as non-odp, negligible gwp, non-flammability and non-toxicity, despite the drawback of high working pressure. practically, co2 have been used for cascade cycles co2/propane and co2/ nh3. as direct expansion cycles utilizing transcritical co2, heat pump water heaters were commercialized in manufacturers that commercialized co2 compressor and its applied heat pump water heater since 2001. however, not so many studies were reported on transcritical co2 cycle that is applied for refrigerator or freezer . for years, authors studied the application of transcritical co2 cycle for refrigeration. as an example, the application for commercial refrigeration equipment was pursued, because non-hfc technology is demanded and the non-flammability is considered to be seriously important as they are often placed in public and asked safety for. in this paper, the idea of transcritical co2 refrigeration cycle with rolling piston type 2-stage co2 compressor for middle range evaporating temperature refrigeration equipment is presented. cooling performance and power consumption of co2 cycle are compared with those of conventional r134a cycle.