The development of system based on natural refrigerants has been very important in these recent decades, in order to mitigate global warming, pollution, but also against environmental risks and those related to health and safety. Water is undoubtedly one of the most abundant and safest refrigerants on the planet. Nevertheless, existing studies on water as a working fluid are few and often theoretical. They are mainly limited by the unavailability of a suitable compressor on the market, making experimental study of this refrigerant complex. A prototype of small capacity two-stage high-speed centrifugal compressor has been used in this study to explore R718 refrigeration cycles in medium temperature applications (cold water from 7°C to 25°C). In this paper, we are studying R718 in a direct exchange mechanical vapor compression refrigeration cycle with and without assistance of a two-phase ejector. The experimental bench is presented and the performances of the refrigeration cycle with and without two-phase ejector are analyzed.