Ahar is name of a historical city located in northwest of Iran. The existing documents and historical writings along with some inscriptions, architectural and archeological monuments within the city and its suburbs all indicate the historic antiquity of this city. From the pre-Islamic period, little information is available about Ahar. Some sites such as the Jame mosque of Seljuk period and Sheik Shahab Aldin Ahary’s complex indicate that it was a developed city in the Islamic period. The aim of this investigation is the identification of the factors affecting on the urban evolution process in different historical periods. The spatial organization of Ahar has been developed within the Islamic period, but the hypothesis of research is the indications of pre-Islamic period are in the present-day location of it. The question of research is: what were the components affecting the formation and development of the spatial-urban organization of Ahar? This investigation is a basic research which utilizes the descriptive-analytic method based on the analysis of historical documents’ contents. The necessary information has been obtained through library and field studies. The research results show the following: the formation of Ahar dates back to the pre-Islamic period. Based on some historical documents and narratives, the location of the initial core of the city is considered to have been in the vicinity of the historical graveyard of the city; and since the reign of Islam, the city has developed around the core, yet at some periods of time in the course of history, the city has undergone locational changes and has developed toward the northern and western grounds.