Recently, aqueous method has attracted lots of attention because it enables the solution-processed metal oxide thin film with high electrical properties in low temperature fabrication condition to various flexible devices. Focusing the development of aqueous route, many researchers are only focused on metal oxide materials. However, for expansive application of the aqueous-based metal oxide films, the systematic study of performance change with process variables for the development of aqueous-based metal oxide insulator film is urgently required. Here, we propose importance of process variables to achieve high electrical-performance metal oxide insulator based on the aqueous method. We found that the significant process variables including precursor solution temperature and humidity during the spin-coating process strongly affect chemical, physical, and electrical properties of AlOx insulators. Through the optimization of significant variables in process, an AlOx insulator with a leakage current value approximately 105 times smaller and a breakdown voltage value approximately 2-3 times greater than un-optimized AlOx was realized. Finally, by introducing the optimized AlOx insulators to solution-processed InOx TFTs, we successfully achieved InOx/AlOx TFTs with remarkably high average field-effect mobility of ∼52 cm2 V-1 s-1 and on/off current ratio of 106 at fabrication temperature of 250 °C.