Theoretical and experimental studies were made on the thermal performance of a hear pump that used a bare flat-plate collector as the evaporator. The analysis used empirical equations to express the electric power consumption of the compressor and coefficient of performance (COP), as functions of temperature of evaporation at the evaporator and that of the heat transfer medium (water) at the inlet of the condenser. The experimental heat pump had a compressor with a rated capacity of 350 W and collectors with the total area of 3.24 m(2) Around noon in winter the evaporator temperature was found to be about 17 degrees C higher than the ambient air temperature of 8 degrees C, and a COP of about 5.3 was obtained when the water temperature at the condenser inlet was 40 degrees C. These measured evaporation temperatures and COPs were in good agreement with those predicted by the analysis. According to the analysis, the total area of the collectors in the experiment was appropriate for the heat pump system. Also, the l-mm thickness of the collector＇s copper plate used in the experiment could be 0.5 mm with little reduction of COP. The pitch of the tube soldered to the copper plate for the refrigerant flow was 100 mm in the experiment, but the COP would only be reduced by about 4% if the pitch were changed to 190 mm.