Dynamic assessment research, with its roots in Vygotsky's Zone of Proximal Development, is still in its infancy (Lantolf and Poehner, 2004; Poehner, 2008). It came to the attention of scholars to break away from a static, incomplete and unfair form of assessment to the dynamic form of assessment in order to more comprehensively uncover learners' independent and assisted level of performance. In order to further investigate effectiveness of dynamic assessment, this study applies the regulatory scale offered by Aljaafreh and Lantolf (1994) to Iranian EFL learners' writing ability. The learners responded differently to the same type of errors they made in the pretest stage after introduction of mediation by the teacher. The regulatory scale which was applied in the intervention stage uncovered the fact that the individual learners had different developmental levels. The result of the study shows that a DA approach to EFL learners' writing ability can prove useful. (Contains 1 table.)