Computations are performed to examine the effectiveness of mono-disperse water droplets in extinguishing a co-flow, propane diffusion flame by injecting the droplets into the air stream. The calculations show that the droplets entrained into the reaction kernel at the flame base are crucial for extinction. The reaction kernel detaches from the burner rim and blows-off when the droplet concentration is increased to a critical value (extinction concentration). At the critical value, the maximum chain-branching reaction (H-2 + O = OH + H) rate in the reaction kernel was found to be reduced by a factor of 5 in our computations. A large decrease in the reaction rate indicates that the maximum heat generations rate and Damkholer number are too low to sustain the flame, and cause the flame blow-off. Large drops are more effective than small drops, and the extinction concentration of water increases from 10.5% to 15% by mass as the size is reduced from 32 to 4 mu m. This is because of competition between the degree of penetration and the rate of evaporation of the water drops. The large drops penetrate the reaction kernel at the flame base better than the small drops, which evaporate completely before reaching the 600K isotherm located well outside the reaction kernel as shown by our computations. As the droplet diameter is increased further (>32 mu m), the trend will reverse as the evaporation rates get too small, despite increased penetration of flame core by the drops.