The objective of this exploratory study was to investigate vocabulary size of Korean high school learners of English. The relationship between their vocabulary size and performance on English tests was also explored. With 97 Korean high school students of 10th and 11th grades, the Vocabulary Size Test (Nation & Beglar, 2007) was employed to measure their vocabulary size. Their performance on English tests was assessed through a school’s English reading test and the Preliminary College Scholastic Ability Test. Results revealed that 10th and 11th graders knew an average of 3,282 and 3,733 word-families, respectively. These two groups showed a 451 word-family difference in vocabulary size, suggesting that high school students, at least for participants of the current study, gained around 451 word-families over a year. While a large number of students achieved a vocabulary size of 4,000 to 6,000 word-families, a significant number of students seemed to have difficulty in learning the most frequent 2,000 word-families. The study found that the vocabulary size played a crucial role in learners’ English academic performance, most markedly in reading comprehension, in line with previous L2 vocabulary research.