The thermal oxidation characterization and its dynamic mechanism of micron-Al powders in air were investigated by simultaneous thermal analysis. Three kinds of micron-sized aluminum powders were heated up to 1110 ℃ at 10 K·min^-1 heating rate in air. The kinetic parameters of the oxidation reaction were calculated by Satava-Sestak integral method. By analyzing the obtained TG-DTG-DTA curves and using SEM and XRD to observe the oxidation products of different stages, it can be found the thermal reactivity of aluminum has size effect: the smaller the particle size is, the deeper the degree of oxidation is. The oxidation process of micron-Al powder is divided into three distinct stages. During stage Ⅰ, below 550 ℃, the oxidation rate is the lowest, the natural amorphous alumina layer on the particle surface grow slowly. During stage Ⅱ, 550-670 ℃, the oxide transformed into γ-Al₂O₃ which can′t form a continuous shell on the surface of Al particles completely, the oxidation rate increases rapidly at the beginning of the stage Ⅱ, but decreases to the minimum when γ-Al₂O₃ layer completely covers the particle surface again. During stage Ⅲ, 670-1110 ℃, because of the volume expansion of molten Al and the shrinkage of the surface area caused by the transition from γ-Al₂O₃ to α-Al₂O₃, the alumina layer produces cracks or breaks, the oxidation reaction is the most dramatic and produced only α-Al₂O₃ finally. Calculations prove that the smaller the particle size is, the lower the apparent activation energy of the oxidation reaction is, and the easier the reaction is. The most probable mechanism functions of thermal oxidation of the samples is the boundary control model function R₃: G(α)=G(α)=1-(1-α)^1/3 in the temperature range from 550 ℃ to 1110 ℃.