Abstract: A significant amount of research in Second Language Acquisition (SLA) manifests that not all Strategy-Based Instruction (SBI) studies have been beneficial in driving interlanguage development forward. The purpose of the current study was to investigatethe effect of metacognitive vocabulary strategy training on the breadth of vocabulary knowledge among a group of intermediate Iranian EFL students. In so doing, two intact classes at Islamic Azad University of Hamedan in Iran were selected as the participants of the study. The total number of students were sixty students with thity students in each of the experimental and control group. The participants were randomly assigned to control group and experimental group. Vocabulary Levels Test (VLT) as an instrument for measuring the breadth of vocabulary knowledge was administered before (as pre-test) and after (as post-test) the treatment of the study. Both groups worked on the same reading passages and textbook. In addition, the students in experimental group were also taught in meta-cognitive vocabulary learning strategies while the students in control group received traditional teaching without any treatment for 12 sessions. The result of one-way ANCOVA indicated that meta-cognitive vocabulary strategy trainingwas beneficial in enhancingthe breadth of vocabulary knowledge of students.