Hydrogen-generation by the reaction of pure Al powder with deionized water under ambient condition was investigated systematically. The results showed that both nano- and micro-sized Al powders could react with water and generate hydrogen under ambient pressure at mild temperature. The nanometer Al powder almost completely reacted with water at 20 degrees C, most of micrometer Al powders could react with water at a temperature of >40 degrees C, implying that small-sized Al powder could directly be able to generate hydrogen without any activation or modification. A shrinking core model was used to analyze the reaction progress, indicating that the Al-water reaction was controlled by the surface chemical mechanism at the initial stage, and then by the H2O molecule diffusion in the byproduct layer afterwards. The activation energy was calculated, which increased from 64.2 kJ/mol to 88.7 kJ/mol with increasing the Al particle sizes from 98.38 nm to 24.94 mu m. X-ray analyses revealed that the reaction byproducts are bayerite, boehmite or a mixture of them, depending on the reaction temperature. The reaction dynamics mechanisms are discussed. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.