<p>The effect of real-time high temperature and thermal treatment on the mechanical characteristics and crack evolution of granite with different grain sizes (i.e., 0.5 mm, 0.7 mm and 1.0 mm) is investigated by numerical simulation employing a grain-based model, and the impact of initial cracks on thermal-induced strengthening is also examined by integrating random cracks within the model before tests. The results revealed that thermal stress, induced by the mismatch in thermal expansion coefficient between various minerals, is the primary distinction between rock specimens in real-time high temperature and thermal treatment. With increasing temperature, the thermal stress gradually accumulates in quartz minerals under real-time high temperature but releases after thermal treatment. The high local contact force significantly affects the peak stress and crack evolution. Uniaxial compression simulation results demonstrate that progressive accumulation of thermal stress induces degradation in macroscopic peak strength and increase of microcrack density. The grain size controls the ratio of intergranular contacts to intragranular contacts, and leads to an increase in strong contact number in the intragrain and a decrease in strong contact number in the intergrain. The strengthening of uniaxial compression strength in the experiment can be well simulated by controlling the number of pre-existing initial cracks in the numerical model. Our conclusions are beneficial to a better understanding of the underlying mechanisms of thermal damage and thermal strengthening of granite for deep geological engineering.</p>