The performance of water-to-water heat pump system coupled with the ground source vertical heat exchanger is presented in this paper. The CAP program of Florida Heat Pump Co. is used to predict the heat pump performances while the EED program calculates the borehole fluid temperature. It is shown that COPH increases with decreasing the temperature of output water for the operation of heating mode and COPR increases with increasing temperature of output water for the operation of cooling mode. The value of specific heat extraction rate must be moderate to insure the reasonable installation cost of borehole system. With $1^{circ}C$decrease of $T_{wo}$ the average COPH increase is estimated as about $0.06/^{circ}C$(for $T_{wo};=;45{sim}60^{circ}C$ range) while with $1^{circ}C$ increase of $T_{wo}$ the estimation of COPR increase is about $0.13/^{circ}C$(for $T_{wo};= ;5{sim}11^{circ}C$ range) at the specific heat extraction rate of 30W/m.