Abstract: Subcritical crack growth of specimens made of marble and granite was studied in double torsion test using Instron1342 type electro hydraulic servo test machine.The relations of the stress intensity factor KI versus the subcritical crack growth velocity V and the fracture toughness KIC were obtained by the load relaxation method under constant displacement.The behavior of subcritical crack growth was analyzed.The threshold values of subcritical crack growth in rocks were obtained.It was shown that lgKI-lgV relations of rocks measured by this method accorded with linear rules,i.e.the relations between subcritical crack growth velocity V and stress intensity factor KI conformed to power law,which was in good agreement with the Charles theory.The range of K0/KIC for these two kinds of rocks was from 0.706 to 0.870.The test results would provide the basis for studying time-dependency of rock stability.