In this study a ground-coupled heat pump (GCHP) system with single U-bend ground heat exchanger has been used for heating applications. The mass, energy, entropy and exergy balance relations are derived and applied to GSHP system. The performance characteristics of this GSHP system are evaluated in terms of exergetic aspects for city of kerman and the optimum size for ground heat exchanger is calculated. The effect of different rock or soil and water-antifreeze inlet temperature on optimum length has been shown. Results show that there is an optimum length and mass for vertical ground heat exchanger.