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In the political-social thoughts of Iranians, since ancient times, the belief that the king and the royal family are the owners of God's majesty has prevailed. During many centuries, this belief has shaped an aspect of the Iranian ethnic spirit in such a way that the worthiest person to obtain the position of ruler is someone who is from the royal family and of Iranian descent. In such an intellectual atmosphere, the families that intended to rule in Iran, even at the non-national and local level, should have benefited from ethnic legitimacy and eligibility; otherwise, they would not be accepted in the public mind. In this research, with the help of the theory of the history of popular mentality - which deals with the influence of popular ideas on the actions of rulers - in a descriptive-analytical way, the influence of the Iranian worldview and their attitude towards the role of genealogy in the legitimacy of the government in the mind and language of the Ghaznavid and Seljuk sultans - which is symmetrical with the emergence of the Shahnameh and the spread of national epics - was shown. From the results of the research, it appears that this simulation of foreign rulers (Turkish and Arab) to the heroes of Iran's national epic is influenced and inspired by public mentality and public opinion. Ghaznavid and Seljuq sultans took advantage of the native and collective memory of Iranians by confiscating Iranian mythological-epic heroes for the acceptability and legitimacy of their rule and in the meantime, the centrality of Shahnameh and its narrations is completely obvious.
 

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The royal institution in ancient Iran involved implications such as race, splendor, and personality. Ahura Mazda brokerage, construction and development, border protection, justice, and religiosity were the functions of royal institution and developed according to the requirements of the time. However, with the arrival of Islam, the royal institution was initially rejected and discouraged; but after a while, it prevailed on the Arab tribal teachings and became the supreme prototype of governance pattern. Investigation of Iranian rulers after Islam and even during Arab caliphs on whether or not the requirements of the royal institution were responsible for its functions (referred to the duties of caliph or Islamic rulers) indicates the importance of these elements in that period. In some texts of the Islamic period including the letter of Tahir to his son Abdullah, we can find the reflection of the aforementioned royal functions; but due to a change in some of the requirements and functions, the book Siyâsat-Nàma of Nizam al-Mulk is the first text of the Islamic period that has focused on them fully and comprehensively. With emphasis on the central role of the king and the royal institution functions, Nizam al-Mulk tried to change the Seljuk tribal government into the one with Iranian governance pattern. Taking into account an analytical approach, this paper tries to investigate the functions and requirements of the royal institution that is the center of Iranian governance pattern in Siyâsat-Nàma and effort of Nizam al-Mulk for changing Seljuk tribe's attitude.