Secretary of State Hillary Clinton’s visit to Indonesia in February 2009 and Signing ASEAN’s Treaty pf Amity and Cooperation （TAC），indicated a big change in U.S. policy toward Southeast Asia. The Obama administration’s policy toward Southeast Asia has the following characteristics：Increasing US participation in regional institutions and building Asia-Pacific community;with “smart power” rationale，strengthening U.S.-ASEAN relations in an effective combination of its hard power and soft power;enhancing the relationship with Indonesia and Vietnam;modifying Washington’s Myanmar policy，engaging with Myanmar’s officials. The Obama administration’s Southeast Asia policy adjustment has complicated impacts. But of course，there remain serious uncertainties in US policy.