Potassium niobate (KNbO3) fine powders were synthesized by a novel hydrothermal method using supercritical water as reaction medium. Synthesis of KNbO3 powders in supercritical water employed significantly low potassium hydroxide (KOH) concentrations (0.1-0.5 M), which was far less than the very high concentrations (above 6 M) required for conventional hydrothermal method for preparing KNbO3 powders. Structure characterization results indicated that the KNbO3 powders prepared in supercritical water had rhombohedral and orthorhombic structures depending on the alkaline concentration. Most importantly, it was found that the KNbO3 powders prepared in this work exhibited strong intensity of second harmonic generation (SHG) in the measuring of nonlinear optical properties. (c) 2005 Elsevier B.V. All rights reserved.