Abstract: Method for evaluating the time-dependent failure characteristics of rocks by fracture mechanics is presented. It based on the testing of fracture toughness KIc and the relations of subcritical crack growth velocity and stress intensity factor, K1-V. KIc and K1-V relations of five kinds of rocks were determined by the double torsion (DT) method. It has been shown that KIc values are specimen thickness independent. For two types of marble, the KIc Values obtained by DT specimen has been compared with those obtained by three point bending specimens. They are fairly agreed. The Kr V relations of rocks were in good agreement with Charles theory and Hillig-Charles theory. Water accelerates the subcritical crack growth.