A simple theoretical model based on electric-field-assisted growth and dissolution of the barrier oxide has been proposed to understand the mechanisms for formation of a one-dimensional (1D) anodic aluminum oxide (AAO) nanopore array. It is shown that formation of a one-dimensional horizontal nanopore array is the natural result of the self-adjusting effect of anodic oxidation. Because the electric field for anodization is in parallel with an aluminum film, the porous structure is formed on the side of aluminum. When the thickness of an aluminum film approaches the size of a single cell, a single array of nanopores evolves after anodization in a steady state for sufficiently long time. Our experimental results support the theoretical model. A well-ordered one-dimensional nanopore array was obtained at 40 V in 0.3 M oxalic acid solution using the modified two-step anodization procedure.