Over the past few years the issue of fire in transport tunnels has become an important concern. Fire safety is now seen as being one of the important elements in tunnel design. For this reason, many tunnels are equipped with critical longitudinal ventilation systems to control smoke in the event of fire. In this study, tunnel fire simulation is done with open-source software FDS. Also, study on the critical velocity is done in tunnel fire. In this research, the effects of geometrical parameters on the critical velocity is studied. To investigate the effects of geometrical parameters, tunnel slope and tunnel blockage is studied. For the study on the effects of tunnel slope, propagation and behavior of smoke is studied for slopes between -10 and +10 degrees. In positive slopes of tunnel, fire plume is spread to the downstream of tunnel and in negative slopes of tunnel, fire plume is spread to the upstream of tunnel. It is observed that with increasing from negative slopes to positive slopes of the tunnel, the critical velocity decreases linearly with increasing tunnel slop. Tunnel blockage is studied for one vehicle and 30 vehicles. The results of study on vehicular blockage show that the critical velocity decreases with increasing percent of blockage.