This meta-analysis investigated the effects of self-assessment and peer-assessment on English language proficiency, cognitive skills, and self-directed learning in the context of South Korean English education. The study utilized 30 individual investigations, collectively encompassing a broad range of student populations and educational contexts. Via a thorough coding process, key study attributes were extracted and used for subgroup analysis. The primary outcomes revealed a mean effect size of g=.558, indicating a moderate impact of self-assessment and peer-assessment on the students’ learning achievements. The effect size was found to be higher in high schools and universities than in elementary and middle schools, potentially pointing to the influence of English proficiency on the effectiveness of these assessment methods. Moreover, while rater training did not significantly alter the effect size, a positive relationship was identified between grade level and effect size. These findings could provide critical insights for educators and policy-makers seeking to enhance the effectiveness of English language instruction in South Korea and potentially elsewhere. (Kongju National University·Jeonju University)