Regionalism emerged in international politics after the World War II, but the end of Cold War has significantly impacted on global politics and regionalism. It is a new phenomenon and nowadays it is difficult to find a country that is not a member of at least one regional organization. Even some of them are the members of more than one regional organizations and international organizations. Economic Cooperation Organization (ECO) as a regional organization established by Pakistan, Iran and Turkey in1985, but after the Cold War Central Asian countries also joined the Organization and ECO expanded with a new situation. In this study to better explain and analyze regionalism in ECO, nefunctionalism, interdependence and liberal intergovernmentalism theories are examined and used together in the context of globalization. It seems that non of theory alone can explain the perfect situation of the region. The advance of technology and information revolution have considerably accelerated globalization and vice versa. Consequently interdependence amongst nations also has been increasing. Such conditions have affected to rise regionalism as a requirement of nations to promote their interests through regionalism particularly after the Cold War. Thus globalization is a main driver of regionalism, however there is interaction between globalization, regionalism and nation-states. The nation-states can manage to take the best advantages of globalization age and remain as an important actor. In globalization age, economic interdependence between the members on the one hand and interdependence between the regions and countries on the another hand have considerably affected on the shape and progress of ECO. Common historical heritage, culture, social and many other factors also have impacted on ECO. Geo-economic and geostrategic locations of ECO also have had important role in the establishment of the organization. The region is very rich in oil and gas which are the main factors of development. Globalization, growth of technology and economic development of EU, China and India''s have impacted on the requirement of energy. These countries have been suffering from shortage of energy and ECO can help them out their energy crisis. Moreover ECO members are also dependent on manufacturing and technology of advance countries like EU, China and other countries. Thus although the process of regionalism in ECO is not fast like EU, interdependence in energy, trade and many other factors within the region and outside the region can drive an push integration not only in the region but also with the other regions and countries.