The main purpose of this study is to investigate the effectiveness of employing Concept Mapping (hereafter CM) in vocabulary instruction on Iranian pre-university students. This study also seeks to find whether there is difference between male and female students' vocabulary learning when they are subjected to CM instruction. In so doing, an Oxford Placement Test was administered among 120 pre-university students in two high schools in Iran. Eighty students who scored between +1 and -1 standard deviation above and below the mean were selected and randomly assigned into concept mapping group (CM) and non-concept mapping group (NCM) (20 male and 20 female). The results indicated that the CM group outperformed the NCM group in vocabulary learning. It was also revealed that there was no significant difference between males and females. Some pedagogical implications in the EFL context are discussed in the subsequent section.