Geothermal heat has now been identified as an effective renewable energy source due to severe environmental impacts created by conventional fossil usage on global climatic change. However, its wide application has been limited due to the lack of knowledge, particularly of the geothermal conditions of reservoir rocks at elevated temperatures and pressures. Such high temperatures and pressures possibly alter the mechanical properties of reservoir rocks due to the associated micro-structural and mineralogical alterations of the rock mass, which are an important attribute for wellbore stability and stimulation of geothermal reservoirs for safe and effective geothermal energy extraction. This study therefore investigates the stress-strain behaviour under in-situ stress and temperature conditions by conducting a series of high-pressure, high-temperature tri-axial experiments on Australian Strathbogie granite under four different confining pressures (10, 30, 60, 90��MPa) and four different temperatures (RT, 100, 200, 300����C). The effect of temperature on the mechanical behaviour of rock specimens was studied under tri-axial conditions and the corresponding fracture propagation behaviour was observed using an advanced acoustic emission (AE) system. The corresponding micro-structure alteration in granite was observed using SEM analysis. According to the findings, increasing temperature leads to an initial increment in reservoir rock strength and shear parameters followed by reduction, and the trend is aligned with the crack formation pattern of the rock mass. This was further confirmed by the SEM analysis, according to which the rock micro-structure is subject to only minor changes at relatively low temperatures and higher temperatures cause micro-cracks to develop along the rock mass grain boundaries. Furthermore, the conventional Mohr-Coulomb criteria failed to model the stress-strain response of rock under geothermal reservoir conditions, and was therefore modified for the corresponding in-situ conditions.