In recent decades, urban settlements across the world have been rapidly growing and turning into the large cities. This expansion is more severe in developing countries, in that one of the most salient features of urbanism in such countries is inappropriate spatial distribution in urban settlements. The velocity and intensity of population growth in cities and its concentration in specific and limited areas together with its reflection on corporeal and physical structures of cities are among the issues that pose serious challenges for many governments and planners in recent years. The lack of attention to urban system and distribution pattern of the urban centers on the one hand, and rapid-growth of urbanism on the other hand will lead to fragmentation and incoherence of the spatial structures. The estimates show that although urban population will have 72% growth between the years of 2000 and 2030, the constructed urban areas with the population of over 100000 will face a growth rate of 175%.
While, slowing down the growth of metropolises has been considered as the main factor in balancing activities and population by most of the 5-year economic, social, and cultural development plans after the Islamic Revolution, investigating the Iranian urban system clearly illustrates the tendency towards population concentration and accumulation in the capital and large cities in recent decades. In 2006, fifteen million people (32% of urban population of Iran) were living in six Iranian megalopolises. In addition, there were 7 cities with 500.000 to 1 million populations in the country, which together with megalopolises' population accounted for 43% (about 20 million) of country's urban population, leading to numerous issues and problems. The process of urban growth and centralization in Iranian urban system not only has unbalanced habitats' population, but also has resulted in environmental pollution, destruction of orchards and agricultural lands, informal settlements growth, and shortage of per capita public spaces and services in the cities, which all have been comprehensively explored in the present articles.
In the present article, spatial-physical dimensions of centralization of urban system in Iran have been investigated through analytic-descriptive research, using the results of population and housing censuses and Zipf's model, urban primacy index, four-city index, and Gini index.
Paying attention to small and medium cities and land-use plans is essential for urban system reform and achieving balanced urban system.