Highly crystalline gamma-alumina (gamma-Al2O3) nanoparticle was synthesized hydrothermally in supercritical water by using a continuous flow reaction system from the starting reagent of Al(NO3)(3)center dot 9H(2)O. The study was performed under supercritical conditions of water; temperature ranging from 400 to 500 degrees C, pressures from 25 to 35 MPa and the reaction time was as short as 63 ms to 3 s. Products were characterized by XRD pattern, BET surface areas, transmission electron microscopy (TEM), and dynamic light scattering (DLS). XRD results revealed that gamma-Al2O3 particles were obtained at 410 degrees C or higher through the dehydroxylation reaction in supercritical water, while gamma-AlOOH phase was predominant at 400 degrees C. Primary particle size of gamma-Al2O3 was about 4 nm and did not depend on the reaction temperature and time. Furthermore. DLS results revealed that the secondary particle size of gamma-Al2O3 dispersed in water increased with increasing reaction temperature and time probably due to particle aggregation. It is noteworthy that secondary particle sizes of gamma-Al2O3 dispersed in aqueous solution decreased as the Al(NO3)(3)center dot 9H(2)O concentration was increased, since particle aggregation was depressed by high zeta potential with lowering the pH. (c) 2008 Elsevier B.V. All rights reserved.