This study presents a numerical simulation model of vertical ground heat exchangers, considering unsaturated hydro static ground conditions induced by the ground water table fluctuation. Heat transfer in ground and grout is modeled as a 3-D FEM transient conductive heat transfer model, where heat transfer between circulating fluid and heat exchanging pipe is treated as 1-D quasi steady state forced convective elements. To take into account the unsaturated ground condition, soil thermal conductivity and heat capacity which are dependent on the matric suction are applied to the ground elements. The developed model is verified with an analytical line source model which considers a homogeneous ground condition. Finally, parametric studies considering various ground water table conditions are conducted to investigate the influence of unsaturated hydro static ground condition to the mean heat exchange rate of ground heat exchanger. Simulation results considering water table fluctuation show 60~100% of mean heat exchange rate for a saturated soil condition and 125~208% of that for a dry soil condition. Thus consideration of unsaturated soil condition is substantially recommended for more accurate design and performance evaluation for ground heat exchangers.