[en] Large eddy simulation (LES) was performed to investigate backdraft in reduced-scale compartments using the fire dynamics simulator. Mixing-controlled fast chemistry combustion model was adopted in the LES predictions of the backdraft phenomenon. The effect of the opening geometry of the compartment on the critical fuel percentage required for backdraft occurrence was numerically investigated by considering compartments with a door or a slot opening. The LES with the mixing-controlled fast chemistry combustion model provided reasonable results for the gravity current and backdraft development process in the reduced-scale compartments. The predicted results of the critical fuel percentage for backdraft occurrence were in good agreement with those obtained by previous experiments on door and slot opening geometries. The LES also predicted the trend in peak pressure with increasing fuel concentration inside the compartment reasonably well. The oxygen concentration of the entrained airflow inside the compartment affected the peak pressure during backdraft development. The peak pressures for the compartment with a slot opening were higher than those for a compartment with a door opening at the same fuel concentration. The difference in the peak pressures between the two compartment geometries was attributed to the difference in the entrained oxygen concentration caused by different ignition times.