(K, Na)NbO3 particles were synthesized using the isopropanol-assisted hydrothermal method. The addition of isopropanol made it possible to prepare (K, Na)NbO3 solid solutions under a mild hydrothermal condition of 2 mol/L alkali solution. The effects of the K/(K + Na) molar ratio, (K + Na)/Nb molar ratio in the starting suspensions, and annealing process on the phase evolutions of the (K, Na)NbO3 solid solutions were investigated. The phases of the as-prepared particles were investigated by X-ray diffraction technique. The microstructure and composition were analyzed by scanning electron microscope and energy dispersive X-ray analysis. (K, Na)NbO3 solid solutions were successfully synthesized by hydrothermal method with the addition of isopropanol through adjusting the K/(K + Na) molar ratio and (K + Na)/Nb molar ratio in the starting suspensions. The change of morphology is also in agreement with the phase evolution. The phase evolutions were discussed and the formation mechanism of the (K, Na)NbO3 solid solutions was proposed.