Four different-sized pure Al powders were used to react with distilled water at 55 degrees C in a low vacuum, and their effect on the reaction dynamics and hydrogen-generation rate was investigated. It was found that the Al particle sizes not only affect the hydrogen-generation rate but also affect the induction time for the beginning of the reaction. The induction time strongly depends on the Al particle sizes, which obviously increased with the Al particle sizes. The hydrogen-generation rate increased with the decreasing Al particle sizes, because the small-sized Al powder has a high surface area. Mechanism analyses revealed that the long induction time for the large Al particles probably originates from a long time for their H diffusion to reach a saturation concentration in the bulk Al metal. The reaction rate of Al with water could be qualitatively explained by a shrinking core model.