Alumina nanostructures, nanowires, and semicolumn nanotubes with high aspect ratios were synthesized by the chemical etching of porous alumina membranes (PAMs) in phosphoric acid solution. The morphology and structure of alumina nanostructures were analyzed by scanning electron microscopy (SEM), energy dispersive spectrum (EDS), and X-ray diffraction (XRD), respectively. The results showed that the typical features of highly flexible nanostructures were around 20–40 nm in diameter and around 100 m in length. EDS analysis indicated that the nanostructures were constructed by aluminum oxide. The crystalline structure of the alumina nanostructures was amorphous, which was in accordance with that of the PAMs. Furthermore, the morphology of the PAMs was characterized in detail by atomic force microscope (AFM) and SEM. On the basis of AFM and SEM observations, a possible formation mechanism of alumina nanostructures was discussed, and the inhomogeneous dissolution between the triple points and the side walls was considered to be the essential factor deciding the formation of nanostructures.