The relation between institutional evolution and legitimacy is overlooked in the study of international relations. Scholars explained the institutional evolution through the lens of realism, liberalism, yet the concept of legitimacy in impacting international cooperation is neglected. I argue in this thesis that the relationship between legitimacy and evolution. Legitimacy defined as "the belief that an institution ought to be obeyed'" is linked closely to normative change. This thesis will analyze how and under what conditions peacekeeping operations change overtime by examining peace operations in Rwanda and East Timor.