Vocabulary knowledge is an important part of measuring EFL students\u27 overall language ability. Generally vocabulary levels are assessed by measuring receptive knowledge. However, to train language learners to become genuinely capable communicators, it is essential to measure not only receptive knowledge but also productive knowledge. To accomplish this, Laufer and Nation (1995) developed the Vocabulary Size Test of Controlled Productive Ability (VLT-CPA). Unfortunately, this test has proved difficult to use with Japanese learners. This paper investigates the possibility of measuring productive vocabulary size of Japanese EFL learners using the VLT-CPA. Results show that, 1) participants\u27 VLTCPA scores correspond to the 45% mark commonly found when using the Vocabulary Levels Test (VLT), thereby reconfirming the results of Waring (1997). Further, 2) productive vocabulary size of the CELT High-score group and Low-score group examined by the VLT-CPA show an almost 30% difference. Thus we determine that the VLT-CPA can be used with Japanese EFL learners, and be regarded as an accurate indicator of productive vocabulary knowledge. Finally, 3) the High-score group obtained scores of about 60% in the VLT-CPA at the 2,000 word level, confirming the importance of encouraging students to strive for a productive vocabulary of at least 2,000 words, in order to become successful language users.論
