The Sassanian dynasty, which established a strong government in the ancient Iran with the dream of the glory of the Achaemenian dynasty, saw the realization of such an idea in the light of integrating the formal religion with the policy. Although Ardeshir Babakan, the founder of the Sassanian dynasty, integrated Iran after conquering the Arscaian dynasty and the feudality of Iranshahr, stabilizing the new regime was possible only in the light of centralized political and administrative systems and the formalization of the national religion. So, the religion had an important stand in the Sassanian discipline in different aspects of life in the Iranian society. However, at the beginning of the Sassanian period, the Zoroastrianism did not have the necessary capacities as a formal and national religion. The most important factors against the formalization of the Zoroastrianism consisted of lack of a unique religious text, dispersed bases for the Zoroastrianism resulting from the attacks of Alexander the great and the feudality. Therefore, the first kings of the Sassanian dynasty, supported by the Zoroastrian clerics, began some important challenges in this regard and formalized this religion in a fluctuating way. This paper deals with the situation of Zoroastrianism at the beginning of the Sassanide dynasty, challenges of the first kings of the Sassanide dynasty and the Zoroastrian clerics as well as the fluctuation of this religion in the process of formalizing it.