This study deals with numerical analysis of fault slip behaviour within deep faulted geothermal reservoirs during injection and pro- duction of fluid. A coupled approach for thermo-hydro-mechanical process modelling is used to describe and quantify the effects of thermoelastic stress on the slip tendency. The results show that the slip tendency of a fault can increase when the cold fluid front reaches the fault due to thermal stress enhancement. Magnitudes of increase in slip tendency depend on the injection temperature and the dip angle of the fault, and under specific configurations, may lead to a reactivation of the fault.