<![CDATA[Vocabulary mastery is fundamental to English as a Foreign Language (EFL) acquisition, yet many Indonesian secondary school students struggle with retention and productive use due to decontextualized, rote memorization techniques. This study investigated the effect of the Content and Language Integrated Learning (CLIL) approach on vocabulary mastery among seventh-grade students at UPTD SMPN 3 Kec. Harau, Indonesia. Employing a quantitative quasi-experimental design with a non-equivalent control group, two intact classes were selected via cluster random sampling: an experimental group (n=29) taught using a CLIL unit focused on procedural texts about Indonesian snacks, and a control group (n=28) taught using conventional, teacher-centered methods. Data were collected through pre-tests and post-tests measuring receptive knowledge of vocabulary form, meaning, and use. Paired-sample t-test results showed a significant improvement in the experimental group from pre-test (M=57.07) to post-test (M=77.24), t(28)= -11.441, p=.000. Independent-sample t-test results revealed that the experimental group significantly outperformed the control group (M=61.25) on the post-test, t(47.234)=6.133, p=.000. These findings demonstrate that CLIL’s dual focus on content and language, contextualized input, and emphasis on meaningful communication leads to superior vocabulary gains. This study provides empirical evidence for CLIL as an effective pedagogical strategy for enhancing vocabulary mastery in the Indonesian junior high school context, offering practical implications for EFL teachers and curriculum designers.]]>
