The Horizontal Directional Drilling (HDD) method is one of the most effective measures in reducing the installation cost of Horizontal Ground Heat Exchangers (HGHEs) in Ground Source Heat Pump (GSHP) systems. However, the performance of HGHEs using HDD method has not been studied well due to the lack of field application. In this study, we conducted Thermal Response Tests (TRTs) in a field test site located in Saga city, western Japan. A numerical model was then developed in FEFLOW software to simulate heating/cooling operations using the HGHE and to predict the behavior of the heat medium temperature of the GHE. Firstly, a polyethylene pipe of 50A (ID/OD:52mm/60mm) and U-tube pipe of 25A (ID/OD:27mm/34mm) were installed at the depth of 8m and 6m, respectively, using the HDD method. TRTs were carried out using these GHEs. According to the TRT results, the polyethylene pipe showed the lower temperature increase per unit heat exchange rate compared to the U-tube pipe. This result was considered to be attributed to the large surface area of GHE and also the absence of thermal interference between pipes which happens when using U-tubes. Next, the cooing/heating operations were simulated after the numerical model was developed and validated using TRT results. Furthermore, case studies were carried out for different installation depth of GHE. The simulation showed that the heat medium temperature of the GHE installed at a depth of 5m was lower and higher in the cooling and heating, respectively comparing to the GHE installed at a depth of 10-15m. This is because that the change of ground temperature of -5m has the phase difference of about 150 days, providing the good heat source/sink to the GHE.