A transparent porous alumina nanostructure was formed on a glass covered tin-doped indium oxide (ITO) substrate by anodization of a highly pure sputtered aluminum layer. Details of the fabrication and microstructures of porous anodic alumina films are described and a possible mechanism of anodization is outlined. The variation of anodic current density reflects three processes, i.e., (i) anodization of the sputtered aluminum layer, (ii) transition of electrolysis from aluminum to the underlying ITO film, and (iii) electrochemical reactions on the ITO film beneath the anodic alumina film. As all the aluminum is completely anodized, the resultant oxide films on the ITO/glass substrate possess a parallel porous structure (phi80-100 nm, cell size in similar to350 nm) with a thin arched barrier layer (similar to80 nm) and exhibit a high transmittance in the ultraviolet-visible light range (75-100% transmittance 300-900 nm).