Experiment results indicated that the interaction of pool fires and water mist coverage determines the effectiveness of water mist system for extinguishing cooking oil fires. Water mist extinguishes cooking oil fires mainly by surface cooling effect at once, and it not only cooled the oil surface, it also mixed and enhanced the heat transfer between the hot oil and cold oil below to rapidly reduce the oil temperature near the surface, below its auto-reignition temperature and prevents re-ignition. Water