This paper presents the cooling performance of a water-to-refrigerant type ground heat source heat pump system (GSHP) installed in a school building in Korea. The evaluation of the cooling performance has been conducted under the actual operation of GSHP system in the summer of year 2007. Ten heat pump units with the capacity of 10 HP each were installed in the building. Also, a closed vertical typed-ground heat exchanger with 24 boreholes of 175 m in depth was constructed for the GSHP system. To analyze the cooling performance of the GSHP system, we monitored various operating conditions, including the outdoor temperature, the ground temperature, and the water temperature of inlet and outlet of the ground heat exchanger. Simultaneously, the cooling capacity and the input power were evaluated to determine the cooling performance of the GSHP system. The average cooling coefficient of performance (COP) and overall COP of the GSHP system were found to be similar to 8.3 and similar to 5.9 at 65% partial load condition, respectively. While the air source heat pump (ASHP) system, which has the same capacity with the GSHP system, was found to have the average COP of similar to 3.9 and overall COP of similar to 3.4, implying that the GSHP system is more efficient than the ASHP system due to its lower temperature of condenser. (C) 2008 Elsevier Ltd. All rights reserved.