During the COVID-19 pandemic, the use of online instruction through different mobile applications has become more common in education all over the world. Therefore, this research investigated the potential effects of using WeChat-based online instruction on Iranian EFL learners' vocabulary knowledge. To achieve this objective, 44 Iranian intermediate subjects were picked out among 67 students based on the findings of the Oxford Quick Placement Test (OQPT). The subjects were then randomly divided into two equal groups: experimental and control. After assigning the subjects to two groups, they were pretested by a vocabulary test. Then, the experimental group received vocabulary instruction using WeChat application, while the control group received a conventional vocabulary instruction. After the instruction, a vocabulary posttest was given to both groups to assess the effectiveness of the treatment. The gathered data were analyzed through independent samples t-test, one-way ANCOVA, and p.....

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