Introduction
Political power’s structure and its spatial distribution is one of the most important issues in political geography. Iran, with thousand years of civilization, has a centralized state and its causes must be seek in ancient Iran and the roots of the formation of the first Iranian government (Medes) and the trend to centralizethe power.
 
Methodology
This paper deals with the exploration of reasons that influenced the power structure in ancient Iran by investigating the structure and spatial distribution of power in ancient Iran in the Median, Achaemenid, Parthian and Sassanid eras. Researchers claim Iran's geographical location and the existence of a powerful foreign enemy named Assyria were the reason of formation of the first centralized government of Iran named Medes.
 
Conclusion
In the beginning, the power structure was not centralized, but was more like an Oligarchy and this structure gradually and in particular in the Sassanid era turned into a centralized and unitary system, and one of the most important reasons was the influence of missionaries in the power structure. It is also necessary to mention that other factors like geographic area and ripples have been effective in the pattern of spatial distribution of political power in Iran.