Stripped porous anodic film formed on aluminum in chromic acid solution was thinned with an argon ion beam and examined in a high-resolution electron microscope. Diffraction patterns obtained from several different areas of the film showed different degree of crystallization. TEM images of areas where many diffraction spots were detected showed clear lattice images of almost the entire field except for the vicinity of the pore walls. Both from the lattice images, which showed distances and angles between crystal planes, and from the analysis of diffraction patterns, the film was identified as γ-alumina. Other diffraction spots and lattice distances were also found and were presumably caused by the slightly mixed other crystal forms of alumina in the film. Careful observation showed many disordered mosaic fractions in the lattice images of the crystalline film, and even taking into account the effect of crystallization by beam irradiation, this fact suggests that the original film was composed of a mixture of ordered and disordered structures.