The aim of the present study is to clarify the thermal performance of the radiant cooling system with Water Flow on Roof, which employs the evaporative cooling effect on the roof surface. In this paper, the thermal performance of this system is discussed from a point of view of the coefficient of performance, using a test house. Measurement results reveal that the indoor globe temperature of the test room is kept below 28 °C on summer days when the maximum outside temperature exceeds 30 °C. The COP varied from 0.9 to 5, in the experiment periods.