Abstract：Since the strategic importance of India Ocean gradually rose in the 21st century, the Indian Ocean regionalism has gained continuous attention from regional powers. In the context, Australia endeavored to promote the institutional change of the sole ministerial regional forum in this region, from the Indian Ocean Rim Association for Regional Cooperation (IORARC) to the Indian Ocean Rim Association (IORA). Based on the Transaction Cost Model, the article mainly dissects the role of Australia in IORAs institutional change. First of all, the paper analyzes the content of institutional change in terms of membership, issue areas, concentration, flexibility, and the right of control. The institution shifted its main issue areas on maritime regionalism from open regionalism, and increased the level of concentration, while maintaining the decisionmaking process on the basis of consensus and the preference for informal agreement. Secondly, improved transparency of ties between Australia with India and Indonesia, and the rising uncertainty of maritime agendas constitute the driving force for the institutional change. Thirdly, compared with the Asia-Pacific Economic Cooperation, IORA, with a high degree of flexibility and decentralized control, is characterized by low transaction frequency and high heterogeneity among members, which weaken the importance of issue areas concerning trade liberalization. Finally, it is a necessity to get rid of two misunderstandings about IORA‘’s lowlevel institutionalization, namely no utility of weak ties and low effectiveness.