Online discussion activity has become an important part of many language classrooms, and has been employed as an alternative medium for students to communicate in the target language. This paper reports on a research project into the use of online discussions in a writing curriculum for English as a Foreign Language (EFL) students at a University in Thailand. The study investigates the rhetorical structure of the students' writing in online discussion postings over a 16-week course using Systemic Functional Linguistics (SFL). It was found that SFL genre can be used as a framework for the analysis of online discussion postings and such texts are often a combination of texts realising different genres. This study claims that treating the online discussion posting as a macro genre (MARTIN, 1994) is useful to the language teacher and researcher when observing students’ contributions in this particular medium of communication.