Mobile technologies are receiving an increasing attention within diverse educational contexts. However, the scope of research on the affordances various mobile technologies provide to learning outcomes is still limited. The aim of this study was to investigate the effect of using WhatsApp application on vocabulary learning of Iranian EFL learners. Twenty-six female EFL learners at preintermediate level of proficiency were selected through convenience sampling to participate in the study. A

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... ulary test was utilized for ensuring the homogeneity of the learners and then the participants were divided into two experimental (N = 13) and control (N = 13) groups, followed by assessing their vocabulary knowledge. Then, the 14-session treatment began, which included teaching new words using WhatsApp in the experimental group and traditional face-to-face method in the control group. At the end of the course, a vocabulary post-test was administered to both groups. Additionally, an attitude questionnaire was administrated to both groups for examining their attitude towards using WhatsApp and the traditional vocabulary learning method. The results indicated that the experimental group outperformed the control group. In addition, it was revealed that the learners who used WhatsApp for enhancing their vocabulary skill had a more positive attitude toward foreign language vocabulary learning as compared to the other group. The implications of the study for mobile-assisted vocabulary learning are discussed.