The effect of initial gas pressure on Al-water reaction was investigated systematically. It was found that there is a non-monotonic relationship between the initial pressure and the induction time for the beginning of Al-water reaction. The induction time decreases with decreasing the initial pressure at the first stage from atmospheric pressure, then reaches a minimum, and finally increases with further decreasing the initial pressure. The mechanism analyses revealed that the special vacuum pressure corresponding to the minimum induction time is close to the saturated vapor pressure, below which probably there are some vapor bubbles passing or adsorbing on Al particle surfaces due to boiling, retarding the hydration process of Al surface oxide film and increasing the reaction induction time. The present results imply that a suitable initial gas pressure should be chosen for Al water reaction to generate hydrogen. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.