Low-temperature processing based on microwave-assisted hydrothermal synthesis was proposed for deposition of piezoelectric oxoate KNbO3 films. The films were deposited on various substrates at reaction temperature below 200 degrees C using Nb2O5 powder and KOH solution. Epitaxial (100)(c) KNbO3 film with similar to 1.3 mu m-thick was deposited at a reaction temperature of 150 degrees C on (100)(c)SrRuO3//(100)SrTiO3 using 12 mol dm(-3) KOH solution for 80 min, which is significantly faster than conventional hydrothermal process. (C) 2022 The Ceramic Society of Japan. All rights reserved.