The Effectiveness of Using English with Lucy Channel on Youtube to Improve the Vocabulary Mastery of 10th Grade Students at SMAN 3 Mataram

Vocabulary mastery is the foundation of English proficiency, yet tenth-grade students at SMAN 3 Mataram still struggle to remember words, understand their meanings, and use them appropriately in context. This study aimed to describe the implementation of the English with Lucy YouTube video in vocabulary learning and to measure its effectiveness for tenth-grade students at SMAN 3 Mataram. A quantitative approach with a pre-experimental one-group pretest-posttest design was used with 35 students of class X.2 selected through purposive sampling. Data were collected through a try-out, a pre-test, three treatment meetings, a post-test, and documentation. The treatment used the video 100 Most Common English Words (Pronunciation & Example Sentence). Data were analyzed using descriptive statistics, N-Gain, the Shapiro-Wilk test, and the Wilcoxon signed-rank test. The results showed that the mean score increased from 86.23 to 96.57 with an N-Gain of 0.80 (high). The post-test means were 100.00 for form, 96.40 for meaning, and 95.14 for use, with an overall mean of 97.18 (very high). The Wilcoxon test showed a significant difference (Z = -5.109; p < .001; r = 0.86). Therefore, the video was effective. Although this channel is well known, quantitative evidence in the Indonesian senior high school context remains limited; this study offers novelty by providing quantitative evidence of the channel’s implementation in receptive vocabulary learning. Teachers are recommended to use short, structured videos combined with active classroom activities, and future researchers are recommended to apply a control-group design.