Although sociocultural theory has aroused researchers' interest in investigating the social role in language learning, few studies have investigated the possible effect of sociocultural-based teaching approaches on reading comprehension and ways to increase learners' strategic reading repertoire. This study employs dynamic assessment (DA), a sociocultural-based assessment tool, to evaluate its dialectic function of simultaneously instructing and assessing the reading comprehension of beginning-level EFL learners. It first diagnoses EFL beginning learners' reading difficulties and then intervenes with interactive mediation to support their strategic use of the learned reading strategies. Three reading difficulties are focused: (1) finding the main idea; (2) understanding unfamiliar vocabulary; and (3) making inferences. The teacher-learner interactions are presented to show how DA enabled the mediator to diagnose the participant's reading problems and to provide the appropriate mediation according to each individual's needs. Results indicated that (1) DA was effective in assisting participants' reading performance, especially the ones who had lower language proficiency; (2) it could better reveal individual participant's source of reading difficulty. Findings suggest that DA provides a more comprehensive picture than static assessment does. It can be used to complement reading strategy instruction and reading assessment in EFL contexts.