ABSTRACT Vocabulary mastery is a fundamental aspect of English language learning because it plays an important role in supporting students’ language skills. However, conventional vocabulary learning methods may limit students’ engagement and motivation during the learning process. This study aimed to examine and analyze the effectiveness of Duolingo, a mobile-based and gamification-oriented application, in improving the vocabulary mastery of eighth-grade students at MTs Darul Istiqomah NWDI Juet in the 2026/2027 academic year. This study employed a quantitative approach using a quasi-experimental method with a Non-equivalent Control Group Design. The population consisted of all 53 eighth-grade students, who were selected using total sampling and divided into an experimental group (Class VIII A, 27 students) and a control group (Class VIII B, 26 students). The experimental group received vocabulary instruction using the Duolingo application, while the control group was taught using conventional methods. Data were collected through an objective test consisting of 25 multiple-choice items administered as a pre-test and post-test. The results showed that the experimental group obtained a higher post-test mean score of 84.25 compared with 71.10 for the control group. The results of the independent samples t-test indicated that the obtained t-value was 4.82, which was higher than the critical t-value of 2.00, with p < 0.001. Therefore, H0 was rejected and Ha was accepted. Furthermore, the Cohen’s d effect size was 0.92, indicating a large effect. Thus, the use of Duolingo was found to be effective and had a significant positive effect on students’ vocabulary mastery. ABSTRAK Penguasaan kosakata merupakan salah satu aspek fundamental dalam pembelajaran bahasa Inggris karena berperan penting dalam mendukung keterampilan berbahasa siswa. Namun, pembelajaran kosakata dengan metode konvensional dapat menyebabkan keterbatasan keterlibatan dan motivasi siswa dalam proses pembelajaran. Penelitian ini bertujuan untuk menguji dan menganalisis efektivitas penggunaan aplikasi berbasis mobile dan gamifikasi, Duolingo, terhadap peningkatan penguasaan kosakata siswa kelas VIII MTs Darul Istiqomah NWDI Juet Tahun Ajaran 2026/2027. Penelitian ini menggunakan pendekatan kuantitatif dengan desain Quasi-Experimental tipe Non-equivalent Control Group Design. Populasi penelitian terdiri atas seluruh siswa kelas VIII sebanyak 53 siswa yang ditentukan menggunakan teknik Total Sampling, kemudian dibagi menjadi kelompok eksperimen (kelas VIII A, 27 siswa) yang memperoleh pembelajaran menggunakan aplikasi Duolingo dan kelompok kontrol (kelas VIII B, 26 siswa) yang memperoleh pembelajaran dengan metode konvensional. Data dikumpulkan melalui tes objektif berupa 25 soal pilihan ganda yang diberikan pada tahap Pre-Test dan Post-Test. Hasil penelitian menunjukkan bahwa rata-rata nilai Post-Test kelompok eksperimen sebesar 84,25, lebih tinggi daripada kelompok kontrol sebesar 71,10. Hasil Independent Sample t-Test menunjukkan nilai t_hitung sebesar 4,82 > t_tabel sebesar 2,00 dengan nilai p < 0,001, sehingga H0 ditolak dan Ha diterima. Selain itu, nilai Cohen’s d sebesar 0,92 menunjukkan ukuran efek dalam kategori besar. Dengan demikian, penggunaan aplikasi Duolingo terbukti efektif dan memberikan pengaruh yang signifikan terhadap peningkatan penguasaan kosakata siswa kelas VIII.
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