This study analyzes the entry threshold of geothermal heat pump system with paprika farming, where the geothermal heat pump system is being disseminated to replace traditional fossil fuel energy system. The geothermal heat pump system lowers the energy cost considerably and alleviates the pressure of the increase of fossil fuel cost that is severely affected by international energy price. For the investment analysis, we adopt real options approach (RO) instead of net present value approach (NPV), where RO applies the concept of financial option to real assets. The results show that the entry threshold with RO is a lot higher compared to that obtained by NPV. This implies that farmers with paprika farming need to consider the irreversibility and uncertainty in the investment analysis. Sensitivity analysis shows the entry threshold is sensitive to the change in parameters used in this study, such as yield, fuel cost, and investment cost. First of all, with the fifty percent subsidy of investment cost, which is currently available, the entry threshold guarantees the profitability of the investment with geothermal heat pump system. This result is robust to the no subsidy of investment cost, too. The entry threshold, however, is sensitive to the change in fuel cost and quantity increased under geothermal heat pump system.