Dynamic assessment rooted in sociolinguistic theory of Vygotsky has a long history in applied linguistics but has recently been the hot topic of EFL studies. The present study aimed at investigating the difference between applying dynamic and non-dynamic assessment of EFL reading comprehension ability and examining its immediate and delayed effect. 197 Iranian university students participated in this study. The design of the study was quasi-experimental and several t-tests were used for answering the research questions. The results of the study indicated a significant difference between dynamic and non-dynamic assessment of reading ability in favor of dynamic assessment. Also a beneficial effect of dynamic assessment for EFL readers was found and the effect didn’t fade away over time. This beneficial immediate and delayed effect on EFL reading ability can have useful implications for EFL learners, teachers and testers.