The main goal of this research study is to analyze the urban system of Iran during the years 1956 to 2021 to propose a new model. This is done by examining the primate city index of the urban settlement system based on previous models. The study employs a descriptive-analytical approach. Initially, it investigates the population trends and the number of cities in Iran over the specified period, comparing the growth rates of the total population and the urban population to assess changes in population distribution across the country. Additionally, spatial analysis of population distribution is conducted using Hot Spot Analysis and Directional Distribution methods. This article evaluates the advantages and disadvantages of various primate city indices, ultimately leading to the formulation of a new index for measuring the degree of urban primacy. The findings reveal that the percentage of the urban population in Iran surged from 31.41% in 1956 to 75.99% in 2021. According to the Mousavi primate city index, urban primacy in Iran exhibited an upward trend from 1956 to 1986, followed by a decline from 1986 to 1996. However, this trend reversed upwards in 2016, attributed to the rising number of cities and migration to newly established urban areas due to drought and relative stagnation in agriculture. The rank-size pattern further validates the outcomes of the new index.