The present study aimed to find out thematic organization and progression in the argumentative writing of Iranian learners of English, representing two levels of language proficiency, and the introduction section of published Research articles (RAs) of Applied Linguistics. For this aim, 60 articles were downloaded from three journals and also 92 MA and BA students majoring in English Language Teaching and English Literature were selected. Then, three topics were used for gathering data from them. Of the written argumentative compositions, only 67 were chosen for the next phase of the study. These compositions together with the RAs were analyzed based on Halliday’s (1985) model of thematic structure and the revised model of Danes’ (1974) thematic progression patterns. The results of Chi-square suggested that there was a significant difference in the thematic structure of the essays written by MA students and the introduction section of RAs. It was concluded that thematicity can be effectively applied in classrooms to help students in writing. Students will know where they are losing their effectiveness in their arguments due to problems with either thematic progression or thematic selection, or both. The findings of this study can be effectively applied in teaching writing skills.