Highly ordered pore array in anodic aluminum oxide was fabricated by anodizing pure aluminum. The order of a pore array was affected by anodizing voltage, electrolyte temperature, and first anodizing time. A regular pore array with mean diameter of 24 nm and interpore distance of 109 nm could be formed by two-step anodization at 40 V, oxalic acid concentration of 0.3 M and electrolyte temperature of 15 ℃. The measured interpore distance showed linearity with anodizing voltage. The diameter of pores was adjusted by pore widening treatment in a 5 wt% phosphoric acid solution at 30 ℃ after two step anodization. The mechanism of self-arrangement of pores could be explained by the repulsive interaction between the pore walls.