Performance analyses have been carried out for a heat pump cycle using zeotropic refrigerant mixtures of carbon dioxide (CO<Sub>2</Sub>) and dimethyl ether (DME). In order to clarify the characteristics and coefficient of performance (COP) of CO<Sub>2</Sub>/DME heat pump cycle, the concentration of mixture was varied over a wide range. The calculation conditions were established as a hot-water supply system and the calculations were conducted by considering the heat transfer between the refrigerant and heat source/sink water. Because the heat transfer between refrigerant and heat source/sink water strongly affects on the cycle behavior, effects of the heat transfer capability parameter were investigated. The heat pump cycle is formed around the critical point for high CO<Sub>2</Sub> concentration mixtures and it is formed under the critical point for low CO<Sub>2</Sub> concentration mixtures. The COP has the maximum at a certain pressure for each mixture and the COPs of the mixture and pure DME are higher than that of pure CO<Sub>2</Sub>. Operating pressure decreases with increase of DME concentration. The effects of heat transfer between the refrigerant and heat source/sink water on characteristics of the cycle have also been clarified.