This paper proposes an optimum operation method for open loop ground heat exchangers. A series of TRTs and artificial heating/cooling operations was carried out with monitoring temperature in the hole of SCW. The ground temperature naturally increases with depth, but a switch between the cooling/heating mode results in a change in the distribution of ground temperature. The effect of the mode change was evaluated by performing LMTD & CFD analysis for a reduced model with the depth of 150 m. As a result, in the cooling mode, the upstream operation is more efficient than the downstream operation and reduces EWT by 2.26℃. On the other hand, in the heating mode, the downstream operation is advantageous over the upstream operation and increases EWT by 3.19℃. The merit of the optimum operation will be enhanced for the typical dimension of SCW with a depth of 400 ~ 500m. In the future, an open loop ground heat exchanger system adopting the optimum operation with variation in the ground temperature will be used in practice.