The study consisted of an investigation into the argument structures employed in the English academic writing of Japanese native speakers and Australian English native speakers in the Arts (humanities) faculty of an Australian university. In order to investigate naturally occurring written argument structures, an in-depth case-study analysis of a small number of coursework essays was conducted. The complexity of argument structures in terms of the elaboration of individual arguments and the relational links between multiple related arguments of extended persuasive discourse were examined. Consequently, the similarities and differences between the L1 and L2 argumentative structures in the English essays and the nature of argument in English native speaker and Japanese ESL writing were identified. The findings indicate that although there were some differences between the micro- and macro-structures of written arguments in the coursework essays of L1 and L2 students, there were also similarities across both groups of writers. This may suggest that the context of learning plays a role in shaping the argumentative discourse patterns of written texts, which has significant implications not only for L2 writers learning the conventions of English discourse in an academic environment but also for future research investigating forms of written argument.