In this study, the authors examined feasibility of a GSHP which uses energy from expander to drive primary (ground side) pump by a simulation (see Fig. 1). The simulator has ability to represent convectional and diffusion heat transfer in soil. It also can simulate as well as dynamic behavior of refrigerant cycle. It has been revealed from this study that the formation temperature falls drastically in the initial phase of the operation and system performance (COP) is reduced afterwards for a low flow rate by single expander-driven pump. The performance of system has been evaluated for a two primary pump system, where one pump is driven by power output from the expander and the other is by external electricity. We have found that there exists optimum capacity ratio of expander and pump for most effective use of the energy from the expander. It has been also demonstrated that overall electrical power consumption can be reduced a few percentage using this system, although further investigation on more detailed system design and operation style should be made.