This article examines China's motivations for trilateral aid cooperation in the context of its seemingly assertive diplomacy in recent years. Previously known for its preference for bilateral aid delivery, China, however, is increasingly conducting trilateral cooperation with western donor states and UN agencies. By employing constructivism and cognitive learning theories, this paper focuses on two perspectives: China's calculation of national interests and international engagement, and is structured around two case studies: UNDP's advisory role for China on development cooperation, and China-US trilateral aid cooperation. It argues that strategically, China is putting growing emphasis on its identity as a growing great power in the development sector, using trilateral cooperation to build its global image. Technically, China's four-decade long external engagement has promoted changes in its ideas about aid cooperation, thus reinforcing its desire for cognitive learning to improve its aid performance.