Ultra-fine alumina was prepared by DC thermal plasma. The influence of reactor configuration, input power and cooling conditions was investigated on properties such as particle diameter and phase composition. The powders synthesized without reaction tube were observed to be of two forms. One was metastable nano-sized powder with an average diameter of 30 nm due to rapid quenching of the vapor phase. The other form was larger-sized powder with an average diameter of 200nm, and it was considered to be due to quenching of small liquid droplets, which had not fully vaporized in short residence time. With a reacting tube, the powder collected at the reaction tube was dominated by α-phase Al₂O₃, whereas the powder collected at the quenching chamber was mainly composed of intermediate γ- and δ-Al₂O₃. Input power and injection position of oxygen did not significantly affect the powder characteristics. To increase the conversion, aluminum powder should be vaporized completely. Most of the powders synthesized appeared fluffy spherical white powders with particle diameter of below 150nm.