The realization that there is a mismatch between the vocabulary level presented in the Korean National Curriculum, and the required vocabulary size for EFL (English as a Foreign Language) high school learners to take the high-stakes Korean College Scholastic Ability Test stimulated the researchers of the present study to administer an assessment of Korean high school learners’ vocabulary sizes. Measurement of vocabulary knowledge was conducted with the adaptation of Nation’s bilingual vocabulary size test, receptive and productive, by improving construct validity of the items. Learners were tested for the 1st ~ 10th 1,000 word bands to ascertain learners’ vocabulary size at each level. The assessment of vocabulary size demonstrated receptive vocabulary knowledge to be as large as 6,000 words. However, unforeseen rises in the EFL learners’ vocabulary sizes at some word bands were observed, which seem to have emerged from the educational milieu and the predominant focus on receptive lexical knowledge and the testing of them. Suggestions are proposed for the revision of word list of the National Curriculum, which would become the blueprint for controlling vocabulary level in the development of national textbooks of English.