The study of the roles and functions of cities and their developments is an important question in modern geographic studies. The unbalanced spatial distribution of populations, irregular migration to metropolises and imbalances in the distribution of economic opportunities between regions have led to focus on the idea of ​​medium-sized cities as one of the most effective approaches to planning of the territory. Therefore, the purpose of this article is to study the performance of the city of Nasimshahr as an intermediary city in the spatial development structure of Tehran province. The research method is "descriptive-analytical" and the data collection method is based on both library and field methods. Quantitative methods and models have been used to study the role of the city. Economic models show that the city's share of industrial employment is above average in terms of industrial employment. The structural change index shows that the changes in Nasimshahr were greater than those in Tehran province. The results of the share change model show that the service sector has grown faster than the province. Considering the spatial structure, whatever the role of the city, the spatial imbalance in the urban network of the province is obvious. Demographic models show that Nasimshahr has great elasticity to attract the overwhelming population of the province. Therefore, given its strengths and possibilities, it can play a key role in regional leveling as a service and industrial center in the southwest of the province.