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.
