This study tries to investigate the frequency and the functions of L1 used by the teachers and the students, including the reasons for using L1 in English class. A case study is used as the research design, with observation and interview as the techniques to collect the data. The result of classroom observations showed that the teachers used L1 limitedly, while the students used it more dominantly. In addition, L1 use has seven functions for the teachers in conducting teaching and learning process in English class, which cover (1) giving instruction, (2) explaining the meaning of words, (3) explaining complex idea or concept, (4) motivating the students, (5) checking for comprehension, (6) explaining error and (7) maintaining teacher-students reationship. Meanwhile, there are ten functions of L1 used by the students in English class, namely (1) clarifying meaning of English words, (2) finding new vocabulary items, (3) expressing their frustration in learning English, (4) processing complex concept, (5) creating enjoyable learning atmosphere, (6) motivating each other, (7) discussing some problems in accomplishing the tasks, (8) clarifying pronunciation, (9) clarifying spelling and (10) clarifying instrcution. Furthermore, the teachers used L1 based on their consideration that L1 plays some supportive role in language teaching. They also used it to avoid students? confusion and because they believe that using L1 can make the students feel more comfortable. On the other hand, the students used L1 because of their habit, the ease of using communicating their ideas, their shyness and fear of making mistakes due to their lack of English proficiency. In conclusion, L1 in English class must be re-examined and used appropriately in order to support the more effecctive teaching and learning process in order to attain the better learning outcome.
Keywords: first language, English class, EFL context, language teaching and learning