Dynamic Assessment (DA) is an implicit kind of assessment that most teachers and learners engage in during class time. Although there exists a body of research on the promising effects of DA, the studies have not compared the results across proficiency levels. This study examined the effect of interventionist approach to DA on Iranian English as a Foreign Language (EFL) learners' grammar achievement at two proficiency levels of pre-intermediate and high-intermediate. To this end, 58 learners, in four intact groups namely pre-intermediate (31 students) and high-intermediate (27 students), were selected and randomly assigned to two quasi-experimental and two control groups. The data were collected through pretests, posttests, and semi-structured interviews. Using a pretest-mediation-posttest design, the participants in the quasi-experimental groups received mediation for eight sessions, whereas the control group participants were taught conventionally. Based on the results, when treated separately, there was a significant effect for the type of assessment, but not for proficiency levels. However, the interaction effect of type of assessment and proficiency level was significant, which meant that the effect of DA on grammar achievement was higher for pre-intermediate learners than it was for high-intermediate learners. Hence, DA could have a potentially positive and dramatic influence on EFL learners' grammar achievement, although the success of learning varied across proficiency levels.