Legislation, as the most important tool for realizing the rule of law, has gone through two fundamental experiences in Iranian history: first, the constitutional period (1976-1979), which was the founder of the parliamentary system and the separation of powers, and second, the Islamic Republic of Iran (1979-present), which has organized the legislative process based on Islamic jurisprudential principles. This study, using a comparative approach, assesses the challenges of legislation in these two political systems. During the constitutional period, despite the establishment of the National Consultative Assembly and the acceptance of the rule of law, factors such as foreign influence, royal interference, weak functioning of supervisory institutions, and restrictions on the implementation of laws reduced the efficiency of legislation. In contrast, the Islamic Republic system, with its dual power structure based on the will of the nation and the authority of the jurist, supervision by institutions such as the Guardian Council and the Expediency Council, and the presence of ideological criteria, faces obstacles such as limited independence of the parliament, conflict between religious principles and customary needs, and a lengthy legislative process. The results of the comparative study show that the concentration of power and the existence of structural obstacles are common challenges of the two periods, with the difference that in the constitutional period, its origin is mostly the interference of the monarchy and external factors, and in the Islamic Republic, it is due to internal structures and multi-layered supervision.