Deeply rooted in the sociocultural theory of mind by Vygotsky, Dynamic assessment (DA) asserts that mediation is essential for online diagnosis in the classroom. One of the major challenges facing language teachers is the assessment of the learners' Zone of Proximal Development (ZPD) level or diagnosing the amount of mediation or scaffolding they require to achieve their potential level. Ongoing assessment of the learner's ZPD and the tailoring of mediation to fit the learning environment seems to be a vital stage. Dynamic self-assessment (DSA) can be applied for diagnostic purposes in writing classes. In this research, it is assumed that the analysis and comparison of teacher's assessment and DSA will not only indicate their ZPD level or the amount of mediation the learners require but also diagnose their weaknesses and strengths in writing. A quasi-experimental research on 60 sophomore English Translation students in essay writing classes in Islamshahr Azad University revealed that DSA not only significantly affects the EFL learners' writing ability, but also it is incrementally correlated with teacher's assessment through 8 weeks of treatment, and the analysis of DSAs reveals the leaner's' weaknesses and the areas which should be emphasized.