Aluminum particle is a metal fuel applied the most frequently in energetic formulations; thus, it is fundamental to clarify its oxidation or combustion mechanism to make them work more effectively. By means of reactive molecular dynamics simulations, this work focuses upon the atomic details of the direct oxidation of molten Al nanoparticles (ANPs) with O-2, which governs the energy release of Al fuels added in energetic materials, and clarifies the origin of the microexplosion-accelerated oxidation mechanism (MEAOM) mentioned experimentally only. The MEAOM occurs in a hot and dense O-2 environment, and stems from the fast evaporation self-sustained by huge oxidation heat release, and the fast formation of hot spots and voids. High temperature, dense O-2 and small particle size favor MEAOM. And the MEAOM features fast particle dispersion, fast consumption of reactants, fast heat release and pressure increase, much complex reactions and intermediates involved, and rapid formation of hot spots, voids and amorphous products. Our simulation results are in good agreement with the available experiment and simulation reports. Meanwhile, the MEAOM is complementary to the recently proposed chain-like oxide nucleation and growth mechanism at mild oxidation conditions; thereby, it is expected to establish a complete mechanism for describing the oxidation of ANPs with O-2, and guide the better use of ANPs, as well as of other metal fuels. (C) 2019 Published by Elsevier Inc. on behalf of The Combustion Institute.