The current study set out to investigate the effects of concurrent group dynamic assessment (GDA) and cumulative GDA on Iranian EFL learners' development of reading comprehension micro-skills. To this end, a convenience sample of 60 intermediate undergraduate EFL university students that were selected based on the results of a placement test and randomly assigned to two experimental groups participated in the study. The participants met once a week for 90-min lessons in a Reading II class over an academic semester. Each group received a concurrent or cumulative GDA mediation on reading tasks with a focus on activities involving five fundamental reading comprehension micro-skills, namely identifying the main idea, finding inferences, finding supporting details, understanding vocabulary, and finding references. A pretest/posttest procedure was used to compare the two group's achievements. The results of t test analyses demonstrated that both GDA approaches were effective but the students in the concurrent GDA group significantly outperformed those in the cumulative GDA group in terms of micro-skills of identifying the main idea and finding inferences whereas no significant differences were observed for other micro-skills. It could be argued that in concurrent GDA since the secondary interactants expect to be called on at any moment, they are more alert and attentive to mediational exchanges, and this contributes to enriching the micro-skills of identifying the main idea and finding inferences. The findings suggest that concurrent and cumulative GDA approaches when applied to teaching reading comprehension should be viewed as complementary. The concurrent approach works better with activities that require higher-cognitive functions and top-down processes such as identifying main ideas or finding inferences. On the other hand, both approaches could be equally beneficial and interchangeably used in activities that involve lower-level bottom-up cognitive processes such as finding supporting details, understanding vocabulary, or finding references. The findings offer significant implications for classroom practice and subsequent research that are discussed.