This study investigated the effect of using self-assessment on Iranian EFL learners' writing. The purpose of this study was to demonstrate whether using of self-assessment as an assessment method was influential in developing learners' English writing performance generally writing processes specifically. The participants of this study consisted of 48 female learners. The subjects in control group (N = 23) were instructed writing by using traditional method whereas the subjects in the experimental group (N = 25) were instructed by using self-assessment method as the treatment. Data were collected through Self Reporting Questionnaire (SRQ) on writing processes and motivation questionnaire. Test scores and Self Reporting Questionnaire scores were used to evaluate English learners' writing. In order to analyze the data descriptive statistics, independent samples t-test, and paired t-test were implemented. The results obtained from the pre-administration of instruments showed that the two groups were homogeneous and their English writing performance was poor. However, the results obtained from the post-administration showed improvement in English writing performance of the experimental group learners in comparison with control group. Findings also indicated that there is statistically significant increase in the learners' use of writing processes as a result of using self-assessment method and their writing has developed. As a result of this study it is concluded that using of self-assessment as an assessment method is an effective instructional technique and a useful evaluation instrument.