Subcritical crack growth of double torsion specimens made of granite is studied by using Instron1342 type electro hydraulic servo test machine in air and in saturated water. The relations of mode-I stress intensity factor KI and subcritical crack growth velocity V are tested by double torsion constant displacement load relaxation method

and the subcritical crack growth parameters are obtained. The behavior of subcritical crack growth law is discussed under two conditions

and the subcritical crack growth parameters are compared. The results show that lgKI -lgV relations accord with linear rule under two conditions. The lgKI -lgV curves of saturated water specimens locate top left corner of those of air specimens. The slope of curve is smaller

and the intercept is bigger. The subcritical crack growth velocity of saturated water specimen is speeded at same stress intensity factor of air specimens

which implies that water speeds up the subcritical crack growth. The results correspond to practice.