When writing an argumentative essay, writers develop and evaluate arguments to embody, initiate, or simulate various kinds of interpersonal and textual interaction for reader consideration (Wu & Allison, 2003). This is quite challenging for English as a second language (ESL) learners. To improve the quality of their writing, students need to review their draft throughout the writing process. This study aimed to investigate the effect of self-assessment in group writing and how group work improves students’ writing ability. An intact class comprising 22 first-year undergraduates participated in the study.  Data were collected from pre- and post-treatment writing tests, semi-structured interview and reflection entries. The results revealed that self-assessment has a significant effect on students’ writing performance. Group work also enhanced social and cognitive development of the students. This study provides insights into the use of self-assessment in writing class to develop learner autonomy and improve writing ability. Keywords: Argumentative essay, Self-assessment, Learner autonomy, Group writing, ESL learners