The oxidation mechanisms of core-shell aluminum nanoparticles (ANPs) in high-temperature steam and oxygen are investigated by ReaxFF molecular dynamics (MD) simulation. The details concerning reaction heat release, heat transfer, atomic diffusion process, and ANP structure evolution are studied by examining the temporal variations of temperature, energy, atoms concentration distributions and particle structure, respectively. The atomic-level heat and mass transfer processes reveal that for both ANP/H2O and ANP/O2 systems, at the initial stage of oxidation, the heat transfer between ANP and environmental oxidizer is dominant. Thereafter, the reaction plays an increasingly significant role. The heat transfer efficiency of ANP/H2O is higher than that of ANP/O2, while the reaction exotherm of ANP/H2O is lower than ANP/O2. The final particle temperature for ANP/O2 system is much higher than that of ANP/H2O. The diameter of the former is also larger. During the oxidation of ANP, the core Al atoms diffuse outward into the oxide shell, which pushes the shell Al atoms outward and results in the expansion of ANP. The shell O atoms diffuse inward and left a vacant lattice site, through which the ambient H and O atoms diffuse into the oxide shell. © Asia-Pacific Conference on Combustion, ASPACC 2019.All right reserved.