The interaction between a water mist and a counterflow diffusion flame over a Tsuji burner is studied numerically. The governing system is comprised of two parts. The gas phase combustion model adopts the one developed by Chen and Weng (1990). For droplets, their motions are described by Newton's second law. The corresponding droplet heat and mass transfer are considered by using empirical correlations. The PSI-Cell model is used to manage the two-phase coupling. The evaporation of droplets through heat absorption from the flame can generate vapor in the flow field. The vapor dilutes both the oxidizer and fuel concentrations, and the heat absorption by phase change lowers the flame temperature. For 50 μm droplets, the envelope flame is extinguished from the forward stagnation area and becomes a wake flame as the mist flow rate increases to 7%. The wake flame no longer exists when the mist flow rate is greater than 15%. The critical values for transition of the envelope flame to a wake flame and complete flame extinction for 80 μm droplets were 4.5% and 10% separately. Under the same mist flow rate, the smaller the droplet sizes, the stronger the flame. The effect of droplet size is studied by changing the droplet size and the corresponding mist flows with the number flow rate constant. The critical sizes for transition of the envelope flame to a wake flame and for complete flame extinction were 55 μm and 75 μm respectively. Future work is also discussed.