In order to optimum utilization of low permeability oil and gas reservoirs, artificial cracks should be made in rock by using the hydraulic fracturing process. The fracture toughness is the material's resistance against crack propagation, which is a critical mechanical parameter to control hydraulic fracturing. Although fracture toughness is an inherent property of rocks, some factors such as rock anisotropy, the geometric properties of specimens in the laboratory scale, the temperature and loading conditions can affect the fracture toughness magnitude. As far as the anisotropy is the most important property of rocks in reservoirs, which exists in many sedimentary, metamorphic and igneous rocks, this study has focused on the effect of anisotropy on fracture toughness magnitudes in anisotropic schist and phyllite rocks. This research consists of numerical modelings and laboratory tests on SCB specimens. The numerical modeling results represented that the fracture toughness in anisotropic specimens not only depends on anisotropy angle but also it depends on two dimensionless parameters, including Young's modulus anisotropic ratio and apparent shear modulus anisotropic ratio. Furthermore, for various anisotropy angles, the magnitudes of the geometry factors of pure modes I and II (YII and YI) are different in a way that YI decreases with the increase in anisotropy angle. The laboratory studies showed that the lowest and highest fracture toughness magnitudes were achieved for anisotropy angles of 0° and 45°, respectively. Also, the crack propagation path was studied and it revealed that for specimens which were examined for producing pure mode I, just the beginning of crack propagation path was affected by anisotropic layers, but for the specimens with notch angle of higher than zero to produce mode II and mixed-mode, the majority of crack propagation path was affected by anisotropic layers. Furthermore, in order to predict fracture, three fracture envelopes, including linear, quadratic, and elliptic, were fitted on experimental data, and a quadratic fracture envelope was drawn on numerical data. The results showed that the linear envelope was the most conservative envelope compared with others.