Aluminum films approximately 100 nm thick were deposited on silicon substrates (SiO2 / Si) by thermal evaporation in a vacuum. Porous anodic alumina films were obtained in a potentiostatic mode at 20 V in 0.3 M aqueous solution of oxalic acid and 1.8 M aqueous solution of sulfuric acid. The main pore diameter was determined using the ImageJ software from SEM images. An algorithm determining the pore diameter in porous anodic alumina films was developed. In the nanoporous alumina films formed in sulfuric acid at 20 V the average pore diameter was 12.3 ± 0.1 nm. In the case of the oxalic acid electrolyte, the nanoporous alumina films formed at 20 V had an average pore diameter of 14.8 ± 0.1 nm. The obtained results are in a good agreement with the literature.