AbstractHydraulic fracturing is the most widely used primary method for the stimulation of unconventional oil and gas resources. While knowledge of hydraulic fracture parameters is critical to reservoir productivity, predicting the fracture initiation is important for determining the complexity of the fracture network. Subcritical crack growth in reservoir rock plays an important role in forming fracture networks, but the understanding of the influence of fracturing conditions on subcritical crack growth is still lacking. In this paper, the subcritical crack growth behavior of sandstone and shale under ambient air, deionized (DI) water, and slick‐water conditions is studied. X‐ray diffraction analysis is done on the rock samples to determine their mineralogical composition, while tensile strength is measured by the Brazilian test. Then in Load Relaxation mode, a Double‐Torsion test (DT) is carried out under different conditions to study the subcritical crack growth of the samples. To further verify the influence of clay‐rich minerals, a similar analysis is performed on clay‐free granitic rock samples. The results of the tensile strength test and DT test demonstrate that the presence of clay minerals is the main reason for the difference in the subcritical crack growth behavior of shale and sandstone samples. DI water seems to better improve the subcritical crack growth compared with other test conditions. The value of the Subcritical Index (n) is smaller in a slick‐water environment than that in a DI water environment, revealing that slick water can better promote complex fracture propagation.