Dynamic assessment is a procedure in which development is simultaneously assessed and improved with regard to the individual's or group's Zone of Proximal Development (ZPD; Lantolf & Poehner, 2004). This study aimed to follow dynamic assessment and investigate the impact of three types of implicit and explicit feedback on the essay writing of English language learners. The participants of the study (N = 32) were male students at the Bachelor of Science level at Sharif University of Technology and Amirkabir University of Technology. The analysis of the data showed that the learners had different ZPDs regarding noticing and developing writing skills and strategies, and performed much better on writing in posttest than pretest. Findings of this study suggested that (a) dynamic assessment of writing had a positive effect on improving internalization of writing content and organization skills and strategies of L2 learners, (b) the internalization of skills and strategies appeared hierarchically, and (c) teachers' mediation in the form of explicit feedback was the most effective hint within the dynamic assessment procedures.