In the development of design and control methodolgy for hot, dry rock (HDR) geothermal reservoir cracks, a full understanding of the environmentally assisted cracking behaviors of rock under the reservoir conditions is a prerequisite. In this stdy, the significance of time-dependent material degradation is demonstrated, which is induced by the water-rock interactions. In a 350°C, 18 MPa water environment, pre-immersion of 2 inch CT specimens of granite is given for different periods using an autoclave, after which the ultrasonic wave velocity measurements and fracture toughness tests are performed. The results show that the hydrothermal environment causes the time-dependent material degradation throughout the specimens tested, and that the crack propagation resistance of the granite is also time-dependent and decreases with increasing immersion time. Based on the experimental results, an immersion time-dependent crack growth model is proposed and applied to simulate the stress corrosion cracking test which has been conducted in the 350°C, 18 MPa water. It is shown that the proposed model allows us to predict the stress corrosion cracking behaviors under the pressurized high-temperature water environments.