SUMMARY
Vocabulary is unquestionably the cornerstone of language learning. No matter the learners' first language, foreign language, age, or level of ability and skill. Yet, it is scarcely founded studies regarding formal English vocabulary knowledge. This investigation expected to explore the correlation between undergraduate EFL learners’ academic self-concept and their formal English vocabulary knowledge. The quantitative research method has been applied. The participants completed a written vocabulary test in English and filled in an academic self-concept questionnaire. The data were analyzed using simple correlation regression and qualitative analysis from data obtained from the interview. The results show that the undergraduate English as a Foreign Language learners’ academic self-concept has a positive correlation with the learners’ formal English vocabulary knowledge. Based on interview results confirm that illustrates the power of contextual language learning. Vocabulary learning was influenced by the amount of exposure, word self-regulation, general intellectual ability, motivation, and creativity factors that support the learners’ vocabulary knowledge achievement. The results suggested that teacher educators should pay more attention to the enhancement of the power of EFL learners’ academic self-concept to help the acceleration of teaching and learning English as a foreign language. For strengthening the academic self-concepts of EFL students as well as some essential considerations to be made before beginning the development of learners' English vocabulary knowledge are discussed. Last but not least, some pedagogical implications are proposed for EFL teacher educators.