While advantages of self-assessment (SA) for young learners of English have been addressed, the factors that could determine the effect of SA have not been much examined. To this end, this study focuses on learners' English proficiency level and examines the effects of SA on three groups of Korean EFL learners with different proficiency levels. A total of 184 sixth-grade students from seven classes of mixed proficiency levels participated in the study. The students were provided with an 11-week course of English instruction in which they were instructed to self-assess their learning progress at the end of each class. Based on the results of pre-test results, students were classified into three groups of low, intermediate, or high level of English proficiency. It was found that the effect of the SA intervention on language learning was positive, but it was greater for low- and intermediate-level learners than for high-level learners. In terms of student motivation, confidence, and anxiety with regard to language learning, there was a significant positive effect of SA regardless of the learner's L2 proficiency level. Further pedagogical implications are suggested.