The backdraft is one of the hazardous fire phenomena, which has threatened the life of fire fighters and residents. Since the repeated experiment is not feasible and its potential hazard. The FDS(Fire Dynamics Simulator) was used with model-free simulation using a finite chemistry and gray gas radiation model. The reduced-scale compartment was considered with horizontal opening window. A gravity currents that fresh air entered into the compartment and hot fuel flowed out of the compartment were found after opening window. The explosive fuel vapor cloud and fire ball behaviors were simulated reasonably. The temporal evolution of pressure in the compartment were compared qualitatively and quantitatively compared to the previous experimental studies.