In April 1994, UN -sponsored Global Conference on the Sustainable Development of Small Island Developing States (GCSDSIDS) was held in Barbados. This was the first time that UN paid special attention to the problems of the small island states. Association of Small Island States (AOSIS), which consists of 36 small island states and 5 observers in the Atlantic Ocean, Indian Ocean, Caribbean Sea, Mediterranean Sea, South China Sea and the Pacific Ocean, played a key role in this conference. The aim of this paper is to analyze the origin and evolution of AOSIS, and present prospects of interregionalism of small island states. The origin of AOSIS was in Small States Conference on Sea Level Rise in Male, the Maldives, in 1989. This was the first collective action of small island states over the climate change issues which would give them considerable impact. It was in the second World Climate Conference in 1990 that AOSIS was formally set up. Since its establishment, AOSIS has further developed the activities. In 1991, AOSIS held the second meeting during the conference of the Intergovernmental Negotiations Committee for a Framework Convention on Climate Change. The AOSIS member countries decided to take collective approach to the negotiations. One turning point came to AOSIS in the UN Conference on Environment and Development in Brazil in 1992. Framework Convention on Climate Change was adopted in this conference, which partly included what AOSIS insisted. However, the assignment of AOSIS did not end, rather it expanded. Agenda 21, which was also adopted in the conference, specifically mentioned sustainable development of small island states. Based on this, the UN decided to hold GCSDSIDS. AOSIS got a new role as a lobbying body for sustainable development of small island states. AOSIS gained the confidence and credibility by taking initiatives in preparations for GCSDSIDS. The prospects of AOSIS's interregionalism would depend on how effectively it could play a role of intermediator between globalism and regionalism