Ultrapure, nanosized alumina (Al2O3) powders are highly required for high performance Al2O3 ceramics. However, the synthesis of the powders via an efficient and low-cost way is still a challenge. In the present research, we treated commercial gamma-Al2O3 powders via hydrothermal treatment combined with CO2 pretreatment technique. The effect of hydrothermal pressure on the crystal phase, particle size and purity of the treated powders were investigated. In addition, the effect of CO2 pretreatment on the purification of the powders was discussed. Commercial gamma-Al2O3 powders are fully converted to boehmite (AlOOH, a derived form of Al2O3) at a hydrothermal pressure of 3.5 MPa. The boehmite powders reduce to the minimum particle size of 50-100 nm after being hydrothermal treated at 3.5 MPa. CO2 pretreatment has been found to be very efficient in the purification of the powders. The Al2O3 content of the powders after being CO2 pretreated at 1 MPa could reach up to 99.9410% which is much larger than that of commercial gamma-Al2O3 powders (99.5096%). The as-received ultrapure, nanosized boehmite powders are promised raw materials for high performance Al2O3 ceramics. (C) 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.