Abstarct: Pollution caused by increasing population and urban traffics in metropolises has become to a national dilemma. Researches show that rail way transport system is the best solution in order to decrease the air pollution in Iran. Hence, constructing urban tunnels is the only way to develop railway system. Considering the costs of tunnel constructing, identifying the risks ahead of tunneling and evaluating the stability and safety related issues are essential in all three steps of design, implementation and operation. As a result, reaching the continuous development in transportation infrastructures could be achieved. Consequently, recognizing, evaluating, managing and controlling the uncertain hazards should be assessed. In this thesis, information of Line No.7 subway tunnel of Tehran has been used for stability analysis of subway tunnels in operational step. To this purpose, firstly, 22 parameters affecting on tunnel stability have been identified. Secondly, Utilizing the rock engineering system (RES) method, Stability index has been introduced. This index has been calculated for 24 tunnels in line No.7 subway system of Tehran. The results show that, zone No.16 (located between Dr. Fatemi St. and Nasr Bridge) and zone No.20 (located between Shahid Hemmat highway bridge and Sanat Sq. (or Shahrak e Gharb)) include higher stability index and could be used as passive defense constructions. On the other hand, zone No. 12 (located between Helal e Ahmar St. and Navab Safavi highway) and zone No. 17 (located between Nasr bridge and Shahid Javad Fazel St.) have had low stability index.