The present paper attempts to examine the possibilities of the Classroom Language Assessment Benchmark (CLAB) as a professional development tool for EFL teachers in Japan. Two questionnaire surveys were carried out several months after the last day of the graduate course (both in 2006 and 2007) where CLAB was used as a self- and peer-assessment tool of teachers' classroom English in their trial lesson.The questionnaire results show that the Classroom English Observation Program (CEOP) using CLAB and the Classroom Language Assessment Sheet (CLAS) is a useful approach for raising trainees' awareness of their classroom English and helping them to strive for improved classroom language proficiency, suggesting that CLAB should be used not as a tool for their qualification assessment but as a tool for their professional development. The paper concludes that an improved, simplified benchmark and assessment sheet should be developed so that when used together they would serve as an important means for language teachers' professional development. (Contains 4 tables.)