The English teacher of SMA Muhammadiyah 5 Yogyakarta used code mixing in the learning process. This research aims to find out what are the types of code mixing used by the English teacher and also to identify what are the factors of using code mixing in the classroom. The researcher observed one male English teacher in using code mixing during teaching and learning process in class XI IPA at SMA Muhammadiyah 5 Yogyakarta. This research used descriptive qualitative and for the data collection method the researcher used observation. The finding shown that the English teacher used the three types of code mixing named inner code mixing, outer code mixing and hybrid code mixing. However, there was one type that commonly occurred named outer code mixing for the factors the English teacher used all of the types named factor of code mixing for constructing and transmitting knowledge, factor code mixing fo classroom management and factor of code mixing for interpersonal relation. In the types of factors code mixing the researcher found that the English teacher commonly used was the second factor named factor of code mixing for classroom management.