Interaction of water mist with cooking oil fires is studied experimentally and theoretically. A LDV/APV system is used to measure the velocity and diameter of water mist at different pressures in the experiments, and the effect of water mist velocity and diameter on fire extinguishment efficiency is investigated. The experimental results show that water mist has excellent surface cooling effect; it can control and extinguish cooking oil fires quickly without re-ignition.The critical temperature (Tfo) is calculated by energy balance equation, and the fire plume momentum is calculated and compared with that of water mist in order to determine the critical velocity (Vwy) of fire extinguishment. This paper provides references for cooking oil fires extinguishment with water mist.