Oxidation is a universal process causing metals’ corrosion and degradation. While intensive researches have been conducted for decades, the detailed atomistic and mesoscale mechanisms of metal oxidation are still not well understood. Here using in situ environmental transmission electron microscopy (E-TEM) with atomic resolution, we revealed systematically the oxidation mechanisms of aluminum from ambient temperature to ~ 600 °C. It was found that an amorphous oxide layer formed readily once Al was exposed to air at room temperature. At ~ 150 °C, triangle-shaped Al₂O₃ lamellas grew selectively on gas/solid (oxygen/amorphous oxide layer) interface, however, the thickness of the oxide layer slowly increased mainly due to the inward diffusion of oxygen. As the temperature further increased, partial amorphous-to-crystallization transition was observed on the amorphous oxide film, resulting in the formation of highly dense nano-cracks in the oxide layer. At ~ 600 °C, fast oxidation process was observed. Lamellas grew into terraces on the oxide/gas interface, indicating that the high temperature oxidation is controlled by the outward diffusion of Al. Single or double/multi-layers of oxide nucleated at the corners of the terraces, forming dense γ’-Al₂O₃, which is a metastable oxide structure but may be stabilized at nanoscale.