The performance of a hybrid cooling system that combines a screw water chiller with a ground source heat pump (GSHP) was measured and analyzed at various cooling loads. In addition, the hybrid cooling system in a building was modelled sophisticatedly using EnergyPlus and then validated with the measured data. The coefficient of performance of the GSHP was lower than that of a conventional chiller in the monitored building, but the hybrid cooling system helped to stably provide the required cooling capacity at high-load conditions. The mean bias error and the normalized root-mean squared error of the predicted cooling load of the building were -8% and 12.4%, respectively. The hybrid cooling system was simulated by varying four operating parameters: the operating schedule, chilled water temperature (T-cw), dry-bulb temperature (T-DB), and entering water temperature (T-EW). The Tcw is ascertained as being the most effective control parameter in the hybrid cooling system. (C) 2010 Elsevier Ltd. All rights reserved.