The fabrication parameters for highly ordered nanopore arrays formed by self-organized anodization on aluminum have been investigated. To fabricate ordered nanopore arrays in alumina layer a two-step anodization process was used in oxalic acid solution. The regular nanopore arrangement, interpore distance and pore diameter in alumina surface were strongly dependent on the applied potential at anodization. The interpore distance and pore diameter increased with anodic applied potential, and hexagonally ordered pore arrangements in alumina were obtained by anodization at 40V. For the effects of the heat treatment of aluminum substrate on size of domains with same pore orientation, the domain size increased with increasing annealing time.