The present heritage designation systems face challenges of inflexibility, accumulation of heritage, and outdated understanding of significance, leading to a burden for both preservation management and sustainable development. This thesis discusses the flexibility issue by focusing on heritage delisting policies, comparing the systems of UNESCO's World Heritage List and the National Historic Landmarks in the United States. It aims to establish a better understanding of the benefits and liabilities of heritage delisting and how the policies can be improved to better function as tools to advance the interest of preservation. The research studies the common processes and impacts of the threatened list and delisting, the requirements and implementation of heritage monitoring and reassessment, and the possibility of adjusting delisting practices to respond to changing conditions or significance. The study identifies problems and potential benefits of delisting in the two systems, demonstrating their different focuses, and how delisting can influence preservation policy. Overall, the thesis provides a reference for future policy amendments and contributes to a more dynamic future designation system.