The recognition of principal stresses in an analysis of engineering constructions which are built on rocks or inside them is essential to the design of structures and their stability analysis. Various methods are suggested due to in-situ stresses determination, and one of the most essential methods is Hydraulic Fracturing (HF), which can be performed in the boreholes. The classical approach is used due to the analysis and determination of the principal in-situ stresses. However, this approach ignores defects such as pre-existing micro-cracks and cracks which exist inside different rocks. Therefore, the fracture mechanics approach was mentioned as an alternative for in-situ stress estimation. In this research, influential parameters on the fracture mechanics approach and stress estimation are analyzed by four analytical methods, which were presented by researchers, and an extended numerical code based on the Displacement Discontinuity Method (DDM). Pre-existing crack length is one of the most important fracture mechanics parameters, which has a major impact on stress determination. In this research, the length of pre-existing cracks was determined by the visual iection method in three micro, millimeter, and macro scales, and also with Rummel and Winter analytical method (1982), the back-analysis method of Zeng et al. and some experiments done by researchers. The length of pre-existing cracks for Azad Pumped Storage Power Plant () differs from 0.1-15.7 mm, and the deterministic length was assumed 4 mm, which was determined by Rummel and Winter method. The results showed that the stress magnitudes of the fracture mechanics approach were the same as the magnitudes of the classical approach. However, they were often more than that of the classic results. The stress results of the numerical method were closest to the classic stress results compared with other fracture mechanics methods. Besides, Abou-Sayed et al., Khosravi et al., and Zeng et al. maximum horizontal stress magnitudes of Azad site were in good agreement with each other. Abou-Sayed’s method showed lower changes in maximum horizontal stress magnitudes as the pre-existing crack length decreased, compared with other fracture mechanics methods, which showed high reliability of this method. The stress magnitudes of Rummel and Winter method were higher than all other methods, and therefore, it is a conservative method in stress estimations. Sensitivity analysis showed that the fracture mechanics approach is eligible to be used for stress determinations during a specific range of estimated pre-existing cracks length. The extended numerical code considers both crack inclination angle and fluid pressure distribution along the crack length, which resulted in a better estimation of maximum horizontal stress magnitudes. The pressure distribution along cracks was an effective item on stress magnitudes and stresses can be doubled. Finally, the maximum horizontal stresses of Azad site varied between 12.1-17 MPa with the classic approach, although the fracture mechanics approach stresses changed between 10.4-25.8 MPa, which covers classic stress magnitude