In recent years, the amount of migration, especially towards large cities and megacities, has been increasing considerably, contributing to overpopulation and density. It is necessary to study which areas are exposed to emigration and which are exposed to immigration. The general goal of this paper is to estimate the number of migrants moving from or attracted by urban regions in Iran based on Zipf's rank-size model and the Gravity model. For this purpose, data from the 1365, 1375, and 1385 censuses have been used, and statistical software SPSS and EXCEL were employed for data analysis. Findings show that Tehran has been the Prime City in the decades under study. This city has been gradually losing its status as the dominant prime city. The immigration rate towards Tehran has also shown a decreasing trend. The migration absorption capacity of some capital cities with regard to their distance to other cities has increased, and others have shown a decreasing trend. In general, during these decades, the effects of distance on the migration process have decreased, while those of cultural, social, and economic factors have been increasing.