Ultrafine AlN powders were synthesized by carbothermal reduction nitridation method under vacuum (VCRN) in a double temperature zones vacuum furnace with alumina and carbon black as raw materials. The mechanism of the process was investigated. The results show that when the soaking time was 1.5 hat 1650 degrees C, 300 Pa N-2 pressure, the purity ultrafine AlN powders were obtained. By analyzing the results of density function theory (DFT) calculation and XRD, the mechanism of VCRN can be identified as Al2O3 -> Al2OC/Al4C3 -> Al6C3N2/AlN -> AlN. The results of theoretical study further proved that the compound of Al6C3N2 can be nitrogenized quickly through the reaction of Al6C3N2 + 2 N-2(g) -> 6AlN + 3C. (C) 2020 Elsevier B.V. All rights reserved.