We proposed a simple method, thermally purified solution (TPS) process, to lower the fabrication temperature of solution-processed hafnium oxide (HfOx) gate insulator films. The TPS process is proceeded by transferring thermal energy to the HfOx precursor solution BEFORE film formation. The TPS process could induce chemical reactions including pre-hydrolysis by thermal energy in the HfOx precursor solution. They reduce the required thermal energy for forming a solution-processed HfOx film and improve the film quality. As a result, the HfOx film fabricated with the TPS process obtains satisfactory dielectric characteristics: breakdown electric field of ~6 MV/cm, leakage current density of ~10-9 A/cm2, and dielectric constant of 10.23 at low temperatures, i.e., below 200°C. Solution-processed indium oxide (In2O3) thin film transistors with HfOx gate insulator with TPS process exhibits field effect mobility of 3.67 cm2/Vs, on/off current ratio of 9.8 × 106, and subthreshold swing of 0.18 V/dec, respectively.