The key aims of the international cooperation among national libraries are to promote better and easier access by more users to national libraries and to improve efficiency and effectivity of national libraries's operation and services. The article describes the international role of national libraries, the cooperative organizations of national libraries and their cooperative activities, and the achievements of UNESCO and IFLA to facilitate the cooperation of national libraries. To discuss the cooperative forms includes 3 kinds: (1) bilateral cooperation (2) area cooperation (3) global cooperation. The part of cooperative activities provides introduction about 4 areas: (1) UBC (2) UAP (3) ILC (4) PAC.