The present study was conducted to investigate the effect of using vocabulary flash card on Iranian pre-university students' vocabulary knowledge. The participants of the study comprised 50 female learners. They were randomly assigned into two homogeneous groups each consisting of 25 learners. The control group received the traditional treatment while the experimental group received the vocabulary flash card treatment. Before starting the treatment, two similar tests were prepared as the pretest/posttest to find out students' vocabulary knowledge at the beginning and at the end of the study. Analysis of the results in the posttest revealed significant differences between the two groups. The results showed the students in the experimental group outperformed the students in the control group in their vocabulary knowledge. Hence, it was concluded that the contribution of vocabulary flash card in teaching vocabulary to students led to a higher level of vocabulary improvement.