The injection of cold fluid into a hot dry rock (HDR) formation causes temperature changes that often induce thermal stress. This will alter the reservoir properties of the rock mass and the fluid, making thermal stress central to understanding the behaviour of HDR geothermal reservoirs. In order to explore the effect of thermal stress on reservoir properties, this paper presents a new numerical model of an enhanced geothermal system (EGS) using a coupled thermo-hydro-mechanical (THM) model. Nine reservoir properties are examined: permeability, aperture, stiffness, pore pressure, effective stress, shear stress, density, viscosity, and temperature. The results show that thermal stress enhances each of these properties substantially when compared with a model without thermal stress. It is clear from the simulation that exclusion of the thermal stress effect in a model could end in an erroneous description of the system output.