Potassium niobate KNbO3 particles having nonlinear optical properties were successfully prepared via a hydrothermal process. When KOH, with a concentration of 8 M, reacted with Nb2O5 particles at 200°C, monophasic KNbO3 was obtained. The high concentration of KOH was found to be a critical requisite for the formation of KNbO3. Compared with the traditional solid-state reaction, the hydrothermal process significantly lowered the temperature required for synthesizing KNbO3. The added amounts of Nb2O5 in the hydrothermal reaction greatly influenced the morphology and particle size of KNbO3, as well as the intensity of the second harmonic generation.