Theoretically drawing on Vygotsky’s sociocultural theory of mind (SCT) and following a sequential exploratory mixed method design, this study probed into the impact of Group Dynamic Assessment (GDA) on the short and long-term Iranian high school students’ grammar ability. Also, a focus group interview was conducted to explore students’ attitudes toward concurrent GDA. This study used a convenient sample of 42 Iranian third-grade students in a private high school who prepared to participate in the Iranian University Entrance Exam (IUEE). The students of the two groups (i.e., GDA and Non-GDA) followed the same procedure (i.e., DIALANG test, pre-test, three conventional teaching sessions and one enrichment session, focus group interview (for GDA group), post-test and transcendence test). Quantitative findings using three independent sample t-tests and two repeated measure ANOVAs revealed that the GDA group significantly outperformed the non-GDA group regarding grammar ability and could apply them in more demanding circumstances. Besides, the thematic analysis of qualitative data showed that the concurrent GDA assisted students to improve their grammar ability. The study's findings highlight the importance of applying GDA as a mediational procedure that assists students in developing their grammar ability in L2 contexts. The findings of the study may assist L2 teachers to apply GDA procedure in their classrooms to save time for teaching and assessing grammatical structures.
