ABSTRACT In an effort to improve integration, ASEAN countries agreed to reduce economic development gap as the goal of equitable development. Economic development gap in ASEAN can be seen from several indicators such as GDP, HDI, the percentage of poverty, infrastructure development and different economic development among ASEAN countries. ASEAN efforts in reducing this gap is done by providing assistance to countries that are still lagging behind. The aid does not only come from the internal ASEAN but also from countries outside ASEAN, for example Japan. This research will discuss the strategy of ASEAN diplomacy toward Japan in efforts to reduce economic development gap in ASEAN. The method was qualitative method, the sources were taken from newspapers, magazines, books, some related documents, and articles. As the result, ASEAN used regional development cooperation strategy (RDC) as a basic framework for all agreements agreed with Japan. Various initiative forums also formed by ASEAN within the framework of the RDC to make Japan as a donor for several projects in the forums. Gaining leverage parallel with Japan, ASEAN used the framework Dialogue Partners (DP). DP did not put both parties as donor-recipient, but rather as dialogue partners. However, overall progress achieved in the forums is still under the expectation and still needs some improvements. ASEAN can make improvements in its diplomacy with Japan so that Japan can give their assistance to the maximum amount and it can be absorbed effectively by ASEAN members. Keywords: Dialogue partner, ASEAN economic development gap, regional development cooperation, ASEAN diplomatic strategy