Both North Korea and Iran have tried to develop nuclear weapons. North Korea nearly completes the development of nuclear weapons and missiles, and is regarded as a de facto nuclear state in the international society now. However, Iran has not consistently developed nuclear weapons, and halts its nuclear-weapon program after reaching the Joint Comprehensive Plan of Action (JCPOA). This study tries to explain why North Korean and Iranian nuclear-weapon programs show different results by focusing on their domestic political structures. To answer this question, this study employs the selectorate theory that analyzes domestic political structures with the selctorate, the set of people who can play a role in selecting the leader, and the winning coalition, the subset of the selectorate whose support is necessary for the leader to stay in power. Based on this theory, we argue that North Korea has the low ratio of the winning coalition to the selectorate and the small size of the winning coalition that allow the Kim leaders to consistently seek their own interests through developing a nuclear weapons program. However, Iran has the high ratio of the winning coalition to the selectorate and the large size of the winning coalition that make the Supreme Leader and the President consider public opinions on its nuclear policy despite the Iranian Islamic political system. This political structure of Iran makes the Iranian nuclear program less consistent. This study’s analysis implies that domestic political structures significantly affect pursuing a nuclear-weapon program.