This study explored how the GRE Analytical Writing Section Discussion Forum, an informal online language learning community in China, functioned to support its members to improve their English writing proficiency. The Community of Inquiry (CoI) model was used as the theoretical framework to explore the existence of teaching presence, cognitive presence, and social presence in the GRE Analytical Writing Section Discussion Forum. The transcript analysis of postings in the GRE Analytical Writing Section Discussion Forum was used to find the existence of teaching, cognitive presence, and social presence, and an adapted CoI survey was sent to members to measure their perceived teaching, cognitive, and social presences. The results showed strong evidences of teaching presence, cognitive presence, and social presence, and high levels of perceived teaching, cognitive, and social presences in the GRE Analytical Writing Section Discussion Forum. The well-designed technological environment, distributed teaching presence shared by moderators and members, and extensive evidences of social presence in the discussion forum worked together to support learning in the GRE Analytical Writing Section Discussion Forum.