Little research has been done on the vocabulary size of Japanese EFL senior high school students chiefly because it is very difficult to determine their vocabulary size accurately. There have been many methodological problems to solve. This paper mainly examines the average vocabulary size of first- to third-year students at a senior high school and the differences according to school years and grades. It also examines the average vocabulary size of second-year students at several senior high schools and the differences according to schools. The results showed that the average vocabulary size of first- to third-year students was about 2900 to 3100, about 3200 to 3500, and about 3700 to 3800 respectively. There were statistically significant differences for first- and second-year students according to school years, but there were no differences for third-year students. The differences were statistically significant according to grades. The results also showed that the average vocabulary size of second-year students at several senior high schools was 2788.12 to 3792.79. There were statistically significant differences according to schools.
