Abstract: Due to the chemical activity of ultrafine powder, it is difficult to obtain reliable results by analysing the constitutents of ultrafine aluminium nitride, especially main phase quantity, nitrogen element. The deviation of the values measured mainly caused by the oxidation and deliquescence of ultrafine AlN. With X-ray diffraction and FT-IR analysis and with the characteristic oxygen release curves under programed heating conditions, the existing state of oxygen in ultrafine AlN could be determined and the method for evaluating main phase quantity of ultrafine AlN is put forward. The results of the evaluation have shown that the main phase quantity of ultrafine AlN powder prepared by the special powder manufacture, DC plasma nitridation of metallic aluminium have reached 95%.