After collapse of the bipolar system and disintegration of the Soviet Union, it was forecasted that all institutions and mechanisms related to the system would disappear, but the two security pacts, Warsaw and NATO, which had emerged out of the Cold War, encountered two different destinies. Warsaw Pact was disbanded following the disintegration of the USSR, but while the same destiny was expected for NATO, it succeeded in overcoming its identity crisis by redefining its functions and duties and justified its existence in 21st century. A pact that seems meaningless, while the United Nations is the custodian of stability and security in the world, was not only able to survive by structural and semantic changes, but act as the executive arm of the United Nations and the world police while the Security Council suffers from an executive vacuum. The change in the concept of security may have played the most important role in this survival. In the post Cold War era we are witnessing the formation of approaches in international relations which are focusing on redefining the concept of security. So, the main question of this research is how NATO defines the concept of security in the new world order and how it has structurally modified itself to coordinate with this change. Copenhagen School as one of the most prominent schools of the field redefined the concept of security by offering a comprehensive interpretation of the concept of security and expanded the scope of security to multiple dimensions. Accordingly, after the Cold War, the nature of the threats has changed and they have become more widespread and diverse. In this regard, by the change in the concept of security from hardware and unilateral dimension during the cold war to software and multilateral in the post Cold War era, we are seeing changes in the functions of NATO from a mere military institution to a political-security institution.