(K0.89Na0.11)(Nb0.85Ta0.15)O-3 thick films were epitaxially grown at 200 degrees C on (001)La:SrTiO3 and (001)(c)SrRuO3//(001)SrTiO3 substrates by hydrothermal method, and their crystal structures and electrical properties were investigated. Film thickness increased with deposition time and reached 6 mu m in 10 h. High-temperature X-ray diffraction measurement showed that successive phase transitions from orthorhombic to tetragonal and from tetragonal to cubic phases take place at 120 and 400 degrees C, respectively. Microstructure analyses were performed by using electron microscopy, which revealed the existence of two types of stripe patterns with a width of 100 nm or less. In addition, scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy elemental mapping showed that Nb/(Nb + Ta) ratio of the deposited films abruptly changed around 700 nm in thickness. Annealing at 500 degrees C led to the reduction in leakage current density from 10(2) to 10(-5) A/cm(2) at 30 kV/cm, showing that annealing is an effective way to improve insulation. Relative dielectric constant (epsilon(r)) decreased linearly with increasing frequency, reaching 450 at 10 kHz. Polarization-electric field hysteresis loop and field-induced stain curve were measured by piezoelectric force microscopy, which showed remanent polarization (P-r) of 30 mu C/cm(2) and piezoelectric constant (d(33,PFM)) of 70 pm/V. These results demonstrate that (K,Na)(Nb,Ta)O-3 thick films with superior electrical properties can be fabricated by the low-temperature deposition technique.