The oxidation and ignition of aluminum nanoparticles with a mean diameter of 150 nm are investigated with the help of simultaneous thermal analysis, X-ray powder diffraction, energy dispersive X-ray spectra analysis (EDS) and scanning and transmission electron microscopy at heating rates of 2-30 K min(-1). A unique early ignition reaction is observed when the heating rate is ≥8 K min(-1) and there is a co-existence of various polymorphs of alumina (γ-, δ-, θ-, and α-Al2O3) below the melting temperature of aluminum nanoparticles. It is proposed that such an early ignition reaction is due to a combined effect of solid phase transformation of the alumina shell and the early melting of the aluminum core, and is responsible for the co-existence of various polymorphs of alumina at the low temperature. The ignition temperature increases approximately with the increase of the heating rate. Regardless of the heating rate, the oxidation scenario can be described by a three-stage reaction with the main reaction occurring before the melting of aluminum nanoparticles.