Different from the oxidation kinetics of other nitrides, the oxide layer on AlN can easily reach tens of micrometers at a temperature above 1200 ℃. In the present study, the oxidation mechanism of AlN is investigated through microstructure observation. The analysis indicates that the oxide layer is full of small pores. The formation of pores generates additional surface area to induce further reaction. The reaction thus controls the oxidation in the temperature range from 1050 to 1350 ℃. The oxidation rate becomes slow as the oxide layer reaches a critical thickness.