Self-assessment has gained much attention in recent years owing to graving emphasis on learner independence and learner autonomy and self-assessment has significant pedagogic value (Mrudula, 2002). The present study was an attempt to investigate whether self-assessment impact Iranian EFL learners' writing skill. To fulfil the purpose of the study, 73 participants from 105 students of advanced level of English in Zabansara and Marefat Institutions were chosen by means of a TOEFL test. Two groups; namely, experimental and control groups, were formed. A writing pre-test was administered to measure their writing ability. The subjects in the experimental group enjoyed the treatment. After each writing activity, they assessed themselves with writing checklist. Three raters scored the activities by utilizing the analytical method of scoring. The teacher commented on the students' weaknesses at the bottom of each checklist. At the end of the study, all subjects took writing post-test. By utilizing an independent t-test, the researcher found out that the change in the experimental group was significant. Therefore, the null hypothesis of the study was rejected. The results of this research can be used by those involved in the field of language teaching.