Metro systems are being recognized as an effective and efficient way to solve traort problems in congested cities as new lines are being planned and built throughout the world. In designing a metro system, fire safety is one of the high priorities in view of the volume of passengers being traorted. It is even more vital if the system is built underground as fire incident in an underground train-way considerably different from fire in a building in term of fire development, evacuation and rescue operations. For a metro train fire, it is important that the emergency tunnel ventilation system is able to control the direction of smoke movement in order to provide a clear and safe path for evacuation of passengers and to facilitate fire-fighting operations. In a tunnel, the predominant method for achieving this objective is for the emergency tunnel ventilation system to develop sufficient longitudinal airflow movement past the fire site so that all the smoke and hot gases will be forced in an opposite direction of evacuation. This is usually achieved by operating the emergency tunnel ventilation system in a push-pull ventilation mode whereby ventilation fans at one end of the tunnel will be in supply mode and the other end in extraction mode. In this project in order to study the formation and spread of fire and smoke in the tunnel, simulation is conducted on a real scale tunnel. First, in order to validate the results of simulation of fire in the tunnel, using the commercial code CFX, simulation is done on a small scale tunnel. Different turbulence models were used for simulation. It is inferred from the simulations that the k-? model and SST model yield results with acceptable accuracy. To assimilate the effects of buoyancy, simulations are performed based on the "Full Buoyancy" model. Moreover, the effects of buoyancy on turbulence production and dissipation terms in the equations of k and ? have been considered. Results show that the use of buoyancy terms in the turbulence production and dissipation terms enhances the accuracy of flow prediction significantly. Using Heat-Release-Rate(HRR) instead of combustion modeling predicts higher temperatures near the heat source. Simulation of one of the metro lines in Esfahan district has also been performed. The considered case is an underground tunnel with a length of 778.2 meters. In this simulation, a maximum heat release rate of 11.9 MW is considered. During the fire, thermal plume is formed due to high temperatures beneath the train as a result of combustion (heat release). The thermal plume first moves upward but then mixes with the incoming cold air, which results in reduction in its temperature. The calculated critical velocity of 2.93 m/s is consistent with the experimental data. Before the ventilation fans start to work, the fire smoke penetrates more than 200 meters upstream. After 1200 seconds, the downstream area is quite affected by the ventilation flow. Although temperatures in these areas are still higher than 50°C, but considerable temperature drop is predicted everywhere, compared to the situation with no ventilation. Comparison of the predicted temperature distributions shows that except at points close to the fire, temperatures predicted by the DTRM radiation model are lower than those where radiation effects are neglected. It may be inferred that radiation have a significant impact on reducing the temperature of tunnel environment during fire. Keywords Numerical Simulation of Fire and Smoke, Tunnel, Heat Release Rate, Critical Air Velocity .