Perovskite-type KNbO3 powder was prepared by hydrothennal reaction using Nb2O5 in KOH solution. A single phase of KNbO3 was obtained when the molar ratio of KOH/Nb2O5 was above 20 and the reaction temperature was above 160 degrees C. Three types of KNbO3 powder with the orthorhombic, tetragonal and cubic symmetries were obtained, depending on the reaction temperature and the ratio of KOH/Nb2O5. The molar ratios of K/Nb in the cubic and tetragonal phases were 0.91 and 0.94, respectively and that of the orthorhombic one was 0.98. and the mass loss was observed in the TG curves of tetragonal and cubic phases. The tetragonal and cubic phases were stabilized by OH- and adsorbed water. (c) 2006 Elsevier Ltd. All rights reserved.