Longitudinal ventilation system is a frequent method of tunnel ventilation which is commonly implemented by use of jet fans. In most studies on longitudinal ventilation application, the effect of fire on its upstream is neglected and a constant-velocity flow is considered in the tunnel entrance. In this paper, fire consequences on ventilation system performance are investigated. It is noticed that airflow induced in tunnel declines as the fire intensifies. High reductions up to about 50% are observed and are attributed to two main phenomena; elevated downstream resistance and fire plume blockage. These effects are combined to form a parameter named fire pressure loss (FPL). An analytical model is also developed that gives acceptable predictions of FPL. Results illustrate that FPL increases with time due to gradual heatup of tunnel walls. Moreover, examination of jet fans operation shows that their efficiency declines when they get very close to fire upstream as a result of jet tilting and smoke stratification disruption. For fire downstream jet fans, the performance is deteriorated to an extent affected by installation height. Also, verification of flow patterns with experimental data is presented throughout the paper.