Vertical closed-loop groun-source heat pump systems have been installed widely in Korea since they can extract a moderate temperature level of geothermal heat in a relatively small area. For a ground heat exchanger, a vertical closed-loop type with brine circulation is mostly preferred since it is simpler and less harmful to the ground environment. However, it requires a secondary loop between the refrigerant in a heat pump and the brine. By adding a geothermal heat exchanger in the secondary heat exchange loop, circulation pumps should be attached and the temperature difference between refrigerant and ground would be increased, which causes performance degradation. In this paper, performances of direct expansion ground-source heat pump were evaluated mathematically as an alternative to the conventional secondary-loop ground-source heat pump.