This study analyzes the bureaucratic politics that emerged in the decision-making process of Korean security policy. Specifically, this paper focuses on the case full participation in the PSI, promoted during the Lee Myung-bak administration, was postponed three times. To explain this, the study presents an analytical framework that sets the bureaucratic control of president and the positioning of bureaucratic organizations as independent variables. As a result of the analysis, The Lee Myung-bak administration’s delay of full participation in the PSI came from two reasons. First, the influence of the Ministry of Unification, which has expertise in inter-Korean relations, was strengthened in the policy-making process. Second, the strength of the president’s control over the Ministry of Unification was weakened as the Unification Minister put more emphasis on the ministry’s interest. In the end, this study is significant in that it analyzes Korean security policy from a theoretical and empirical perspectives.