Abstarct: Designing and implementing large underground structures, geometric shapes selection and section size, stability analysis and design of their support system, are subject to geomechanical properties and stress conditions of the region. With the Rock mass resistance properties, the design and implementation of structures on rock or in rock will be less problematic. To achieve this important, or should be determined directly and by performing field tests, the resistance properties and the rock mass deformation, or by using the results of the instrumentation and using the methods of back analysis, the rock mass resistance parameters are evaluated. Using the methods of back analysis can be very important and less costly than performing direct experiments. In this thesis, the main purpose of using the back analysis method is to evaluate the geomechanical parameters (design parameters) in access tunnels to the Cavern Power station of the Azad dam Project, using the instrumentation data. To do this, firstly, the geomechanical and environmental parameters obtained from the laboratory tests results in the modeling of two stations from the access tunnel sections using the FLAC2D software, Which has been selected from the main access tunnel (MAT) station and the next station of the Transformer cavern Access Tunnel to the head of the power station cavern (T4). Then, baxsed on the data obtained from the instrumentation (convergence) and direct return analysis with Alternative Univariate search Method, the values of the four geomechanical parameters including the modulus of elasticity, the ratio of the stresses, the internal angle of friction and cohesion are estimated. The results for the MAT access tunnel, 8.2 GPa, 1.2, 42 degrees, and 1.85 MPa, and also for the T4 access tunnel, are estimated at 9 GPa, 1.7, 43 degrees and 1.7 MPa, respectively.