The study examines the Japanese EFL students' vocabulary size at the 1,000- to 4,000-word levels, using my newly developed vocabulary size estimation test (hereafter VSE test). The study also examines the rank correlation in the success rate in answering in my test. Data are analysed in terms of the response of knowing the meaning of a word in my previous checklist tests and the JACET List of 8000 Basic Words (hereafter JACET 8000), and according to participants' grades and schools. Results show that the average vocabulary size of students is much smaller than previously estimated by existing estimation tools. There is a statistically significant difference in the means in the vocabulary size according to grades and schools. There is a relatively strong rank correlation between the response of knowing the meaning of a word and the success rate in answering while there is a relatively weak rank correration between the rank in the JACET 8000 and the success rate in answering. Also there is a moderate to strong rank correlation between grades and between schools in the success rate in answering at each level with few exceptions. I conclude that educators should recognize the fact that there are many words difficult for students to acquire in common regardless of grades or schools and should change or devise the way of teaching these words.