Abstarct: The research presents a mexta-analysis on the effectiveness of content and language integrated learning (CLIL) on students’ language proficiency. By establishing a set of inclusion/exclusion criteria, a total of 22 primary studies with 76 different effect sizes were extracted from different master's thesis, doctoral dissertation and articles in 2007–2017 and in English. Four moderator variables including language skills and components, subject matter, educational level and publication type were identified. It was found that the overall effect was g= 0.81 which represents a medium effect size with respect to Plonsky and Oswald’ (2014) scale. As a result of moderator analysis, the highest effect sizes are in the master's theses. In addition, CLIL has the most effect on students’ grammar and listening proficiency and in lower levels of education especially in elementary schools. It also has the highest effect when combining with hotel management as the content. Afterward, the study concludes with a discussion for implication and suggestion for future studies. The findings have clear implications for practitioners, researchers and curriculum developers.