Tunnel fires have escalated over the past years and have resulted in many disasters. Consequently, tunnel fires must be studied and safety regulations and standards must be introduced to limit these disasters. The experimental tests in tunnel fire give critical results about tunnel fire impact, but unfortunately these tests are very expensive and may cause extensive damage to the tunnel structure. Therefore, large-scale modeling using fire dynamic simulation is one of the best techniques used to limit these costs and losses. Fire dynamic simulation was validated and different simulations were performed to investigate the effect of the thermal attack of the fire on the tunnel. Parametric study of large-scale fire tests in Makkah’ King Khalid Road Tunnel showed that: (1) Increasing heat release rate per unit area (HRRPUA) by 10% raised the radiation by ~30% while decreasing HRRPUA by 5% lowered the radiation by ~50%; (2) Increasing Heat of combustion by 50% raised the radiation and CO and 〖CO〗_2 concentrations by ~17% and ~20%, respectively; (3) Increasing CO-yield by 75% raised CO concentration by ~50% while decreasing CO-yield by 75% lowered CO concentration by ~75%; and (4) Changing soot-yield affected all parameters except the oxygen concentration and air velocity and the most parameter affected by changing soot-yield was the radiation where it was decreased by ~40% with increasing and decreasing soot-yield by 75%. The simulations proved also that increasing ambient temperature from 10.5 °C to 40 °C raised the maximum temperature from 85 °C to ~110 °C and raised the maximum radiation from 30 kW/m2 to ~38 kW/m2. In addition, increasing tunnel height from 9 m to 12 m lowered the radiation by ~50% while the temperature was almost unaffected by changing.