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Highlights • Epitaxial (K 0.88 Na 0.12)NbO 3 films were prepared by hydrothermal method. • Effect of crystallinity of Nb 2 O 5 source to deposition behavior was investigated. • Deposition thickness increased by suppression of powder formation. • Efficiency of source to film can increased. Abstract Epitaxial (K 0.88 Na 0.12)NbO 3 films were deposited at 240 °C onto (1 0 0) c SrRuO 3 //(1 0 0)SrTiO 3 substrates utilizing a hydrothermal method. Both crystalline and amorphous niobium oxide powders were utilized as starting niobium sources with KOH and NaOH as potassium and sodium sources, respectively. Film thickness increased with deposition time, but eventually became saturated. The saturated film thickness of (K 0.88 Na 0.12)NbO 3 films prepared from the amorphous niobium source was greater than that from the crystalline source for mixtures of 6 and 7 mol/L KOH-NaOH solutions. This is found to be due to the suppression of the (K,Na)NbO 3 powder simultaneously prepared with films by selecting amorphous niobium source. Their electrical and piezoelectric properties were nearly identical after following heat treatment at 600 °C for 10 min in an atmospheric O 2 flow irrespective of the starting niobium sources; The average values of remanent polarization and effective longitudinal piezoelectric constant, d 33 , were 5–6 μC/cm2 and 45 pm/V, respectively. [ABSTRACT FROM AUTHOR]