In the early stages of designing a rock or soil structure, laboratory and in situ tests with proposed imperical tables and diagrams, are useful. During the construction, deformability of soil/rock is measured using different in-situ monitoring instruments and input parameters of modeling are updated. Among the various monitoring methods, estimating of displacement with extensometer, due to its simplicity and explicit expression of structural response, is more attaractive and applicable. Masjed Soleiman HydroElectric Power Plant (HEPP) project in Khuzestan province of Iran is executed on the Karun River. The geological units are a series mudrock, conglomerate and sandstone layers. The main cavern of pahse 1 is constraucted with 30m width, 50m height and 125m length through these layers. There is a potential for sweeling in mudrocks when the water table is cover murdrocks. A direct back analysis modeling is conducted for evaluating the fractured/jointed rock mass mechanical paramaters using measured displacement in extensometer when swelling in mudrocks is occurred. According to the literature parameters of both intact rocks and rock fractures/ineterfaces between layeras may be affected by swelling of mudrocks. In fractured/jointed rock masses when the joints/interfaces are oriented and intersected a undergroung large cavern, the hybrid Jointed Model-Distinct Element Method (JM-DEM) which is considers a discontinuum media as a geometry and facilited a strong numerical tool for simulating this such a complicated media is preferred compared with the simple continuum modeling approaches. Sensitivity numerical back analyses about the effect of intact rock properties which are friction angels, cohesions and also interfaces mechanical behaviors which are normal and shear stiffnesses and surface friction angles and cohesions on deformability of rock mass are run and more significant parameters are recognized and calibrated. The calibrated parameters are used for predicting the deformability of rocks nearby the cavern and evaluating of the unstability of rock blocked. The results show that the deformation is less sensitive to the intact rock parameters and also strength properties of interfaces such as cohesion and friction angles. However both normal and shear stiffnesses of interfaces control the deformability of rock mass which is also reported in the literature. The lowest error function of the results of numerical back analysis models are approximated as 5×10 8 kPa/m and 2×10 7 kPa/m for the normal stiffness and shear stiffness respectively of the contact between mudstone layer with other layers. The results of the prediction models, suggests that the most of the displacements obtained in the right side of cavern roof region and its right and left walls, from allowable values related to the displacement of bolts installed, is exceeded, which shows an unstable situation, in this area. Therefore a more srong support system with long boltlength and large capacity are demanded.