This study investigated the effects of three vocabulary development strategies of keyword method, thematic clustering and developing morphological awareness on the Iranian EFL learners’ vocabulary repertoire. Adopting a quasi-experimental design, sixty high-intermediate to advanced Iranian EFL learners were randomly assigned to three experimental groups and partook six online sessions of vocabulary instruction during a week. Using a pre-test and three post-tests, the vocabulary repertoire of the subjects was measured. The pre-test was administered a week before the commencement of the treatment sessions; the first post-test was administered immediately after each treatment; the second post-test was held 24 hours after each treatment and the third post-test was held one week after the end of the treatment sessions. Paribakht and Wesche (1993) model of measuring vocabulary enhancement was used to quantify the vocabulary repertoire of the subjects. Split-plot ANOVA test revealed that the subjects in the keyword group outperformed the subjects in the other two groups in all the three post-tests. Moreover, it was revealed that the thematic clustering group outperformed the developing morphological awareness group. The findings of this research may have pedagogical implications for English teachers, learners and material developers.
