Water mist has been recognized as an alternative for halogenated hydrocarbon fire suppressants. In this study, the effect of water mist on the methane/air counterflow diffusion flame was investigated numerically. The numerical simulation was performed using OPPDIF code in CHEMKIN package, modified to include the evaporation process of water mist. When the water mist evaporates completely in the flame zone, the evaporation process itself scarcely affects the critical stretch rate at extinguishment. The flame temperature is slightly affected by thermal effect, and it decreases with increase of the amount of water mist added. The simulated stretch rates at extinguishment are in good agreement with previous experiments in the range of the mass fraction of water mist Y0 < 0.060. The largest effect of water mist on stretch rate at extinguishment is attributed to sensible heat, followed by latent heat and chemical reaction. The chemical effect is rather small, but not negligible as compared to the dilution and thermal effects. The chemical effect is attributed to the decrease of radical species in chain branching reactions. The suppression effectiveness of water mist is better than IG-55, IG-541, N2 and CO2.