Energy-extraction from nuclear materials produces high-level radioactive waste (HLW). In geological nuclear waste storage repositories, the decay of radioactive elements generates heat, exposing the reservoir rocks to high-temperature conditions for long periods. To explore the effects of these conditions, this study examines the ability of granite to resist fracturing after thermal treatment for 10 hours, 10 days, 30 days, and 60 days. The results show that: The fracture toughness of the granite remained basically unchanged with up to 10 days of thermal treatment. After thermal treatment for 60 days, the mode-I, mode-II and mixed-mode (I+II) fracture toughness decreased by 15.39%, 18.07% and 15.18%, respectively, compared with samples heated for 10 hours. The change trends of the ability of granite to resist tensile, shear and mixed (tensile + shear) failure with increased thermal treatment duration were basically consistent. Moreover, there was little change in its brittle fracturing characteristics with increase heating duration. Changes caused to the internal microstructure of the granite by high temperature were ongoing even up to 60 days.