The relationship between republicanism and Islamism in the Islamic Republic system is one of the theoretical issues that have always been discussed before the formation of this political system, and due to its content and theoretical depth, it seems unlikely that it will easily disappear from the atmosphere of political debates and doubts.The present descriptive and analytical research is written with the aim of explaining the sociological duality of republicanism and Islamism in the Islamic Revolution of Iran in the thought of Ayatollah Morteza Motahari as one of the theorists of the Islamic Revolution. The findings of the present research show that in the Iranian society after the Islamic revolution, the separation of republicanism and Islamism and the separate investigation of these two is one of the factors of not recognizing this phenomenon and not facing this important issue correctly. In Motahari's thoughts, republic without Islam and Islam without republic cannot be realized. In fact, the legitimacy of the system is related to Islam, and its acceptance is related to the people. He considers "republic" as a form of government so that Islamic rules and laws can be implemented in that format, and he does not consider it to be associated with what is prevalent in the West; Because many of those elements have nothing to do with republicanism as a form of government, but are related to their anthropological, cosmological and sociological foundations.