The mitigation effects of ultrafine water mist on hydrogen/methane mixture explosions with hydrogen fraction (phi) of the range from 0% to 60% were experimentally studied in a vented chamber with obstacles. The spraying time, droplets size of water mist and the volume ratio of hydrogen were varied in the tests, and the key parameters that reflect the explosion characteristics such as the flame propagation imagines, flame propagation velocity, and explosion overpressure were obtained. The results show that the ultrafine water mist presents a significant mitigation effect on hydrogen/methane mixture explosions. The flame propagation structures are similar under the condition of without and with ultrafine water mist while the flame temperature is declined by the physical and chemical inhibition by ultrafine water mist. In addition, the mitigation effect increases with the increase of water mist flux. As a result, the maximum flame speed and overpressure of phi = 30% hydrogen/methane mixture explosion are declined by 33.3% and 58.4% under the condition of spraying for 2 min with 15 mu m ultrafine water mist, respectively. Besides, the mitigation effects of ultrafine water mist on phi = 30% hydrogen/methane mixture explosion descends evidently with the increase of the droplets size of the range from 6 mu m to 25 mu m, which due to the easier evaporation and the greater total droplets surface area of the smaller water mist. However, the explosion mitigation effect of ultrafine water mist on the hydrogen/methane mixture actually descends with the increase hydrogen fraction. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.