Abstract: The grain size has a significant effect on the mechanical behaviors of the thermally treated granite and further affects the safe and stable operation of high level nuclear waste (HLW) repository. Therefore, the granite with two kinds of grain sizes under Brazilian splitting and triaxial compression is adopted to investigate the variation of physical and mechanical parameters with confining pressure and temperature. The results indicate that there are more flaws in the coarse-grained granite than in the fine-grained granite, which results in the less strength and elastic modulus in the coarse-grained granite than those in the fine-grained granite, and the strength and elastic modulus in the coarse-grained granite are more sensitive to the temperature. After the peak strength, the coarse-grained specimens show ductile failure under high confining pressure, and the residual strength increases with the temperature. However, the fine-grained specimens show brittle failure under high confining pressure. On the one hand, micro-cracks are induced by high temperature, which results in the reduction of cohesion among grains. On the other hand, high temperature increases the friction among grains. Therefore, the strength first increases and then decreases with the temperature. The confining pressure can restrain the radial strain and decrease the Poisson’s ratio, and partial flaw closure due to the action of confining pressure results in the decrease of the potential of the specimens to adjust when the applied axial compression decreases, which increases the Poisson’s ratio. Therefore, the Poisson’s ratio of thermally treated granite specimens shows different trends with the confining pressure.