Porous anodic aluminum oxide (AAO) film has potential to be a candidate material in nanofabrication, membranes, and humidity sensors. Anodic oxidation is typically preferred to fabricate AAO because it is an inexpensive and controllable process. In this study, a different approach to fabricating AAO by a single step anodizing in oxalic acid assisted by externally applied vertical magnetic field, is adopted. The anodizing experiments were conducted in 0.3 M oxalic acid aqueous solution as electrolyte. The vertical magnetic field was generated by a superconducting magnet with the range of 1 to 5 T. The results of the experiment showed that the magnetic field can promote the anodic dissolution of Al3+ ions, and hence increase the AAO thickness by twice as much as without one. However, the pore diameter was decreased by the increased magnitude of the magnetic field. Interestingly, the AAO formed by anodizing with the vertical magnetic field showed a vertically aligned cylindrical structure of the pores. On the other hand, AAO formed by anodizing without the magnetic field showed random growth of pore structure. The density of AAO pores was also increased by the increase in magnitude of the magnetic field. The deep and straight structure with high density and porosity of AAO is advantageous for further application as template and sensor materials.