This paper applies a commercial field-model code, CFX, to predict the ISO 9705 compart ment fires for three different ventilation rates. The predicted results generally agree well with the corresponding experimental data. The ventilation intensity changes the flow path of entrained air and flow structure on the doorway plane to affect the fire structure inside the compartment and mass flow rate across the doorway. There exist many recirculation cells in the compartment to facilitate the mass and heat transfer. The temperature stratifica tion phenomenon is found away from the burner. The thinnest hot layer is observed in the ventilation rate of 2 kg/s.