Self-assessment (SA) allows students to evaluate their language performance, as well as encourage and stimulate their language learning. However, only a few studies have been conducted on self-assessment in EFL contexts, particularly in Thailand. Among these studies, they have rarely focused on students' perceptions and how they use self-assessment to improve their writing ability. Thus, this study explored university EFL students' perceptions and reflections on the use of self-assessment in English writing in Thailand. A mixed-methods research design was used to explore a total of 41 students in two classes (18 and 23 students). Questionnaires, self-assessment forms, and reflective journals were used to collect data from two groups of students enrolling in writing courses at two universities. Mean, S.D., and dependent t-test were used for the analysis of quantitative data, while content analysis was used for the analysis of qualitative data. The results revealed that these students perceived self-assessment as a tool for writing development (self-monitoring), a means of improving their attitudes about writing, and a means of developing their self-confidence. The results suggest that using self-assessment can help students become independent learners and improve their performance in English writing.