Abstarct: Hydraulic fracturing is known as one of the most important methods of stimulating oil and gas reservoirs. Extensive studies have been performed on its influential parameters such as the effect of horizontal and vertical stresses, stress regime, mechanical properties of the formation, natural fractures of the formation. However, there are some ambiguities about hydraulic fracturing and therefore the need for analysis in this area is felt. In this research, in order to better understand the mechanism of formation and expansion of cracks in the process of hydraulic fracturing and laboratory study of crack development, acoustic emission has been used. Experiments were performed on concrete block specimens with dimensions of 30 cm3 with a central cavity with a diameter of 50 mm under true triaxial stress conditions and acoustic emission monitoring was performed simultaneously with the injection of fluid into the specimen. Experiments were performed under different stresses applied to the specimens to investigate the effect of horizontal and vertical stresses. In order to investigate the crack deflection behaviors in the hydraulic fracturing process and the effect of natural fractures on the formation, pre-cracked specimens have been used. The results of the experiments showed that with increasing the minimum horizontal stress, the fracture pressure increases and with increasing the vertical stress and the maximum horizontal stress, the fracture pressure shows an increasing and decreasing trend. The fractures in anisotropic stress compounds are in the direction of maximum horizontal stress and in isotropic stress compounds are radially random. The pre-manufactured crack in the pre-cracked specimens reduce the fracture pressure of the specimens and the magnitude of the difference in horizontal stresses during the experiments clearly affects the crack propagation path during the hydraulic fracturing process. Also, various acoustic emission parameters have been analyzed and the results show a good relationship between cracking behavior in hydraulic fracturing tests and acoustic emission parameters.