Conditions for a self-organized formation of ordered hexagonal structure in anodic alumina were investigated, using oxalic or sulphuric acid as an electrolyte. Highly-ordered nanopore arrays with pore densities of 9 × 109- 6.5 × l0l0cm-2 and high aspect ratios over 3000 were fabricated by a two-step anodization process. The array exhibits characteristics analogous to a two-dimensional polycrystalline structure of a few micrometres in size. The interporc distance can be controlled by changing the electrolyte and/or the applied voltage. The formation mechanism of ordered arrays is consistent with a previously proposed mechanical stress model, i.e., the repulsive forces between neighbonring pores atthe metal/oxide interface promote the formation of hexagonally ordered pores during the oxidation process.