Dynamic assessment (DA), stemmed from both Vygotsky's (1978) learning theory and Feuerstein's (1979) theory of mediated learning experiences, is an alternative to static assessment. It focuses on both instruction and assessment aiming at promoting learning through mediation. DA has been widely researched in different linguistic areas, but there is paucity of research on its practice in ESP contexts. Accordingly, this study investigated the effectiveness of DA on incidental vocabularies emerging in technical reading textbooks, written for electronic engineering students. The study employed a quasi-experimental research design. Due to sample selection problems, an intact group of 25 BA electronic students were selected from the University of Zanjan. A pre-test was administered to check whether they had previous knowledge of the target words, incidentally acquired during the reading activity. As for the instrument stage, DA procedures were utilized in order to individualize participants' assessment. Following DA implementation, a post-test similar in content to pre-test, was administered to the same participants. The significance of DA for the enhancement of incidental vocabularies was to make participants aware of the strategies of identifying, evaluating and monitoring vocabularies (Nassaji, 2003) through mediation process. The results indicated that participants' incidental vocabulary learning promoted dramatically using DA, which employed structured hints for the mediation process. The results of this study can inform both teachers and learners to provide a step by step procedure to promote both teaching and assessment of ESP learners' vocabulary.