Epitaxial (KxNa1-x)NbO3 films with various compositions x were deposited on Nb-doped (100)SrTiO3 single crystal substrates at 240 °C by the hydrothermal method. Detailed crystal structures were analyzed by X-ray diffraction. All the (KxNa 1-x)NbO3 films consisted of a perovskite phase without any impurity phase. High-temperature XRD data showed that (KxNa 1-x)NbO3 films with x = 0.03 and 0.86 transformed to a single cubic phase at 600 °C through the intermediate phase. On the other hand, (KxNa1-x)NbO3 films with x = 0.24 showed two diffraction peaks at 600 °C originating from individual phases having different unit cell volumes. The result shows that these films consisted of a mixture of two perovskite phases with K- and Na- rich (KxNa 1-x)NbO3 composition.