As it is the case in all the other professions, English teachers need to have competencies in English so called - English proficiency, a major content knowledge for EFL teachers. They should have, part of them is the knowledge of the language i.e. knowledge about English grammar or English usage in order to make teaching workable and realistic. The lack of English language proficiency in the interactional process in the classroom is assumed to be a stumbling block in the successful teaching. This paper aims at informing the result of a study on the English language Usage of Teachers' Spoken language in the Classroom Interaction. The site of the study is the Public Senior High Schools located in Padang that are implementing FISS programmes; and the participants are Physics teachers along with their students. Data of this study is drawn from direct classroom observation, questionnaire and intensive interview; and these data were analyzed quantitatively and qualitatively. The analysis of English language usage was focused on how the speakers, in this study the teachers of RSBI schools handled the basic rules of spoken language in the classroom; in particular, the use of words, such as nouns, verbs, adjectives, Adverbs, and Prepositions. It was found that The English language Usage of Teachers' Spoken language in the Classroom Interaction to some extent is still problematic