It is well known that electrochemical anodizations of valve metals are performed under the normal pressure (1 atm = 0.1 MPa), and only columnar pores could be usually obtained in porous anodic alumina (PAA). So far, there have been very few reports involving the atmosphere pressure and its effect on the nanostructure of PAA. Here, a novel PM nanostructure with hexagonal-prism pores has been successfully fabricated under a vacuum system (0.01 MPa), and its forming process is clarified by the anion-contaminated alumina model and oxygen bubble mold. The present results can provide unique insights into the inherent relations between the structural features and anodizing parameters in all anodizing process. (C) 2013 Elsevier Ltd. All rights reserved.