Abstract: The effect of high temperature on the fracture toughness of the granite (Sui county, Hubei province) which is selected as the candidate of the parentrock for storing nuclear wastes was systematically studied. In the range of room temperature to 300℃, the test result of the variation of the critical stress intensity factors, Kic, of the granite with increasing temperature has been obtained. A threshold temperature, 200℃, was found, above which the fracture toughness of the granite decreases quickly. The fracture surfaces created after tests were reviewed by using an electronmicroscope. The mechanism of the variation of Kic for the granite with increasing temperature has been explained. It is suggested that the threshold temperature be used as one of the important criteria for selecting the granite as a parentrock for storing the nuclear wastes.