The aim of this study was to find out factors causing teachers to do code switching. A qualitative case study was used as a research approach. This research was conducted at SMA Negeri 15 Palembang with two teachers of English as participants using purposeful sampling. In collecting data, the researcher used interviews through open-ended questions. The data were analyzed using thematic analysis techniques. The result of this study based on the interviews showed that there are two factors causing teachers to do code switching. Two factors are linguistic and non-linguistic. Linguistic factors are divided into language stagnation and lack of students’ knowledge about vocabulary. Furthermore, nonlinguistic factors are divided into the emotional condition of the teacher and condition of the class situation. Keywords: code switching, factors of code switching, linguistic, non-linguistic