The Mifil ultrafiltration membrane made from aromatic polyamide PA-100 has been studied using a SmartSPM-1000 atomic force microscope (AIST-NT, Zelenograd, Russia). Pores on the AFM images have been identified using the Gwyddion software in two ways, by threshold and watershed methods. The pore size data have been subjected to statistical processing and interpretation. Along with the log-normal pore-size distribution law, a bimodal curve of a special kind has been considered. The accuracy of the approximation has been assessed using Pearson's goodness of fit test, in which the bimodal distribution has demonstrated better agreement with the experimental data. It has been shown that the watershed method provides more detailed information about the pore size distribution of the investigated membrane.