Abstarct: Now large part of electric energy is producted via hydroelectrical power plants that they can constructed via 2 methods of underground and surface depended on existing conditions. For being underground, it’s necessary that to construct of large scale spaces. So because of being large of dimension, special sensitivity and high life, their design and stability analysis is very important. Stability analysis and problems related to stability of underground spaces that are one of parts of this study, it’s a current and applied subject in Rock Mechanics and Geotechnic. Whereas developement of manifold softwares and advanced computers for repetitive calculations, numerical methods for stability analysis of underground spaces in design phase and excavation phase are used extensively. Hence,using of three dimensional softwares can be useful in design, stability analysis and evaluation of underground spaces support. Dam and Hydro Power Plant (HPP) of Sardasht is one of important civil projects of IRAN that its studies have began several decade ego and its outfit has began at 2008 August. Capacity of designed HPP for this dam is 120MW(3×40MW). Bieng underground or bieng surface of power plant is very important in Sardasht project because both factors such as construction of extensive network of underground excavations (main and transformer caverns, S-shaped shafts, surge tank and etc.), high cost of supporting because of weakness of surrounding rock mass and too topics related passive defense through sensitivity and history of this region, selection of method is very important. In this study, using rock mechanics information and data related to surrounding rock mass, power plant capacity, caverns dimansion and geological investigations that are collected via Mahab Ghodss and Moshanir consulting engineers and Sepasad engineering company and statistical analysis is carry out on them, underground spaces (main and transformer caverns, headraces tunnels and draft tubes) are modeled in FLAC3D software, then underground and surfase power plants are compared viewpoints: stability, cost, technical and passive defense.