A novel anodic oxidation method for aluminum thin-film on the glass substrate is proposed for the fabrication of a gate insulator in thin-film transistor (TFT). A proposed rear-positioned cathode makes for much more smooth surface with an average surface roughness of under 4 nm in comparison with conventional cathode positions. The anodic oxidation conditions were comprehensively investigated. The current density, ethylene glycol concentration, and cathode position were chosen as the elements of anodic oxidation condition matrix, and all these conditions affected the film formation. The surface morphology and nanostructure of the anodized films were characterized with an atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (TEM). Films formed at higher ethylene glycol concentrations of over 50% and higher current densities of over 0.9 mA/cm2 exhibited higher breakdown electric fields of over 7 MV/cm, and lower leakage currents. These films had relatively smooth surfaces and took on the shapes of the underlying aluminum. In contrast, films formed at lower ethylene glycol concentrations of under 50% and lower current densities of under 0.9 mA/cm2 had rough surfaces and microvoids. A high concentration of ethylene glycol increases the cost of mass production, however, the proposed rear-positioned cathode method can be achieved even when the concentration of ethylene glycol is under 50%.