Aluminum nanoparticles (Al NPs), due to their high energy density, are important materials for propulsion systems, material synthesis and hydrogen generation. However, the oxidation mechanism of Al NPs at large heating rate remains inconclusive due to the lack of direct experimental evidence. Here, we studied the oxidation mechanism of Al NPs under large heating rate (on the order of 10(6) K/s or higher) by a simple flash ignition method, which uses a camera flash to ignite Al NPs. The flash ignition occurs when the Al NPs have suitable diameters and sufficient packing density to cause a temperature rise above their ignition temperatures. Importantly, transmission electron microscopy analysis reveals that the Al NPs are oxidized via the melt-dispersion mechanism, providing the first direct experimental evidence thereof. Finally, flash ignition is also applicable to the ignition of flammable gaseous, liquid and solid materials by the addition of Al NPs in lieu of sparks and hotwire igniters. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.