Many people nowadays master more than one language, and because of that code-switching occurs. Code-switching does not only occur in social context, but also in classroom context. This study was conducted in order to find out the code-switching used by an English teacher in a teenagers’ classroom. This study is aimed to analyze the types of code-switching used by the teacher and also under what circumstances the switching occur to identify its function. Using qualitative approach, the writer did this study based on code-switching theory of Wardhaugh (2006) and theory of classroom interaction of Walsh (2011). The writer collected the data from the teacher’s talks in a teenagers’ classroom. The findings revealed that the teacher used both types of code-switching and under the four features of classroom interaction. The types of code-switching were intra-sentential and inter-sentential switching. The function of the code-switching identified through the four features of classroom interaction were control of the interaction to control the participation, content, and procedure of a lesson; speech modification to clarify meaning and to ensure the students’ understanding; elicitation technique to elicit response for checking concept or comprehension; and repair to correct or clarify error directly or get the students to correct the errors. From this study, it could be concluded that the teacher used code-switching in the classroom to make everything clearer for the students in learning foreign language so they would not get confused and to avoid misunderstanding in the classroom.