ABSTRACT Dwi Rahayu Prasetyaningsih, 2017. Exploring the Use of First Language (L1) in English as a Foreign Language (EFL) Classroom (A Descriptive Study at the Tenth Grade of SMA Labschool of Syiah Kuala University). Thesis. Graduate English Education Program in Language Education, Syiah Kuala University. Advisors: (1) Dr. Bukhari Daud, M.Ed. (2) Dr. Siti Sarah Fitriani, S.Pd.,MA. Key Words: First Language (L1), English Foreign Language (EFL) Classroom This study discusses the use of Bahasa Indonesia in English as a Foreign Language (EFL) classroom in Indonesian context. This study aims at (1) figuring out the functions of using L1 at the tenth grade of SMA Labschool of Syiah Kuala University, and (2) finding out the students’ perspectives of the use of L1 by the teacher at the tenth grade of SMA Labschool of Syiah Kuala University. Data were obtained from class observations and interview. The observed class was X IPA 1 classroom. The class was observed three times using a framework adapted from Sali (2014). Video recorder was used during the observations. The interview involved six students from the classroom. The students are those who have high, middle and low achievement. The interview was done in Bahasa Indonesia considering that it helped the students to express easily their opinions without worrying of limited vocabulary. Audio recorder was used to record the interview. The gathered data were analysed qualitatively based on Miles and Huberman’s (1984) steps: data reduction, data display and conclusion drawing or verification. To find out the functions of L1, this study consulted Sali’s (2014) framework in the analysis. This study showed that the first language was used for academic, managerial and social or cultural purposes. Overall, the first language was used by the teacher in many aspects such as confirmation, talking about learning, giving instruction, explaining aspects of English, monitoring, checking comprehension, establishing rapport, eliciting, drawing attention, translating words and sentences, reviewing, drawing upon shared cultural expressions, joking and praising. The confirmation and joking found in this study are not included in Sali’s framework (2014). On the other hand, managing discipline which was included in Sali’s framework (2014) did not occur in this study. Further, this study also indicated that most of students agreed to the first language use in their English classroom as it positively influenced them in learning the target language.