Potassium tantalate powders were hydrothermally synthesized at 400 degrees C using Ta2O5 and KOH as starting materials with various KOH concentrations (0.1-1.0 M) and heating durations (2-48 h). A defect pyrochlore phase of KTa2O5(OH)center dot nH(2)O, was obtained at low KOH concentration and short reaction time. The perovskite phase of KTaO3 predominated as the heating duration and the KOH concentration increased. Pure KTaO3 was obtained in 0.5-1.0 M KOH aqueous solutions and reaction times of 8-48 h. Heteroepitaxitial KTaO3 thin films were achieved on the (100) SrTiO3 substrate in 0.5 M KOH solution under supercritical water conditions. Based on the XRD, SEM-EDX and EBSP (Electron BackScatter Pattern) results, epitaxial KTaO3 crystals were grown on the (100) oriented single-crystal SrTiO3 substrate. Synthesis of perovskite KTaO3 crystals in supercritical water employed significantly low KOH concentrations (<0.5 M), which was far less than the very high concentrations (>7 M) required for conventional hydrothermal method.