Abstract
This research aimed to examine the factors that cause code mixing and to describe the language forms used by teacher and students in daily communication. The method used in this research is descriptive qualitative design. The data are the sentences that contain code mixing in teacher and students' daily communication. Recording and documentation were used to collect the data. The researcher collected the data from the activities of teacher-students in boarding school. Interactive model (Miles and Huberman's technique) was used in data analysis. The findings show that there are three forms of the code mixing: Code mixing in the form of word, phrase and sentence. Further, the teachers and students did code mixing because they are able to use two languages, they want to practice the language that have learned, they don't know vocabularies, phrases or sentences in that language, to be sounded great and cool, they are accustomed to use code-mixing with friends, not to break the language rules of boarding school.