Abstract: A one-zone transient model is developed for obstructed fire where extinguishments primarily occur as a result of reduction in oxygen concentration due to consumption of oxygen by fire and dilution of oxygen with water vapor. The model is based on mass conservation of gas species and conservation of energy. This model is used to predict the fire extinguishments by water mist system under the continuous and cycling discharge modes. There is a good agreement between numerical and experimental results in air temperature and dry oxygen concentration. In addition, results show that cycling discharge water mist system would give more rapid extinguishment and consume less water for the obstructed fire in comparing with the continuous discharge mode.