Models of two-dimensional and three-dimensional subsurface fracture networks are proposed on the basis of ＇＇fractal geometry＇＇ to describe geothermal water flow. Using this fracture network model, as well as the single fracture model and the permeable zone model, a parameter study of the performance of Hot Dry Rock geothermal energy extraction systems, such as the changes in output water temperature and thermal output with time, is performed. The results show a strong dependence of the system performance on geothermal reservoir characteristics, such as the fracture density of the rock mass. The fracture network model proposed here can describe the performance of reservoirs which vary between the single fracture model and the permeable zone model, and is expected to bridge the gap between the two models, It is confirmed that the performance of geothermal energy extraction systems can be predicted as a function of the fracture density of the rock mass by using the fracture network model.