After approximately three decades of implementing population control and family planning policies, Iran's population policy underwent a significant reversal with the announcement of general population policies in 2014. This shift experienced many fluctuations, ultimately leading to a complete change marked by the enactment and implementation of the “Youthful Population and Family Protection Law” in 2021. This study aims to address the question: "Why and how did the national population policy in Iran change?" The study analyzes the policy change process using the Advocacy Coalition Framework (ACF), adopting a post-positivist philosophical orientation, a qualitative research paradigm, and a case study strategy. Utilizing this framework can help improve, strengthen, and address its shortcomings, contributing to the theoretical development of policy studies. The findings revealed the formation of two advocacy coalitions within Iran's population policy sphere: (a) the coalition of population control and family planning advocates, and (b) the coalition of population growth and childbearing advocates. Additionally, the policy shift from population control and family planning programs to the enactment and implementation of the Youthful Population and Family Protection Law can be attributed to the causal mechanisms of this framework, namely "policy-oriented learning," "external shocks," "internal shocks," and to a lesser extent, "negotiated agreements".  Policymakers should aim to improve their understanding of the policy-making process in Iran, recognizing the role and importance of coalitions, and draw lessons from both successful and un successful experiences in policy-making.