[Abstract] Vocabulary is an important element of language proficiency, and acquisition of an extensive vocabulary should be a goal for every language learner (Nation, 2008). As teachers, it is important that we incorporate a focus on vocabulary into our language courses, but in order to do so it is helpful to have some knowledge of our students' current vocabulary level （Beglar, 2010）. The Vocabulary Size Test is a free vocabulary test available online, and is designed to test the level of students' vocabulary, with 140 items designed to measure the first 14000 words of English. This paper presents a Rasch analysis of the first 80 test items used with university students in Japan. Rasch analysis was used to determine the relative difficulty of each item, and also to assess the validity and usefulness of the test in this context. Results show that although the test is useful in assessing the relative vocabulary levels of students, the items do not behave entirely as predicted by the difficulty according to the word levels, and care should betaken by teachers hoping to use the test to ascertain student knowledge of different vocabulary levels.著者専攻: TESOL