Transient simulations with full hydrogen chemistry were performed to reveal the flame structure and extinguishment process of co-flow hydrogen diffusion flame suppressed by ultrafine water mist (UFM). As UFM was added incrementally to the oxidizer stream, the flame experienced a series of destabilization process, i.e., detachment, drifting and blow-off. The simulations predicted that the critical mass flow rate of 10-mu m UFM was 6 g/min, which is in agreement with the value calculated by a perfectly stirred reactor model and the value measured by the experiments. The critical mass flow rate exhibited a plateau region as the diameter increased from 5 gm to 20 gm. The optimal diameter for UFM was 10 mu m. A scrutiny on the extinguishing mechanisms reveals that both the chemical kinetic effect and latent heat play important roles in determining the optimal diameter in this configuration. For the chemical kinetic effect, water molecule inhibits the flame through 1) enhancing the chain-terminating reaction H + O-2 (+M) = HO2 (+M) and 2) subsequently scavenging free radicals in the flame. An energy equation was used to investigate the relative importance of extinguishing mechanisms for UFM. It shows that the thermal cooling outweighs the chemical kinetic effect in terms of contributions to flame inhibition although the chemical kinetic effect is obviously enhanced compared with N-2. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.