One of the achievements of the Islamic Revolution is the dominance of the democratic method in managing the country based on the patterns of political system which are taken from separation of powers thought; a thought that associates other systems and in the light of this thought and regarding the realities in the Islamic Republic of Iran, semi-parliamentary and semi-presidential patterns dominated the country. The present article seeks to analyze the “why” of the acceptance and fluctuation between the semi-parliamentary and semi-presidential patterns and the tendency towards parliamentary system in the Islamic Republic of Iran. (Question) In the Islamic Republic of Iran, choosing any of these two patterns aimed at a better management of and acceleration in the country’s affairs, and now with the same reason, a new tendency is towards that pattern. (Hypothesis) The article seeks to figure out the reasons for the tendency towards the semi-parliamentary, semi-presidential and parliamentary systems. (Objective) The authors believe that answering the question and testing the hypothesis relies on the legal-historical investigation of the events happened after the Islamic Revolution. (Methodology) The appearance of the problems in the scope of the president’s legal authority, legislative power and judiciary along with other basis such as the need for increasing the role of people, set the ground for the tendency towards change in the structure of the Constitution and a desire for parliamentary system. (Finding)