This study analyses the use of the code mix by students in the French School of Post-Masters department. The design of this study is a descriptive qualitative design. Source of the data of this study is the Whatsapp text. The researcher collects the data by screen capture of the text in Whatsapp and transcription. In analyzing the data the researcher used three steps based on Norman in Pitriyani (2007: 21), They are, (1) make the data classification based on types of code mix, (2) make the classification based on the cause factors of code mixing, and (3) find the percentage of each type of code mixing by the Bugin’s formula. The result of this study shows that the type of code mix that frequently occurs is the insertion of words, followed by the insertion of an expression. While the cause of the code mixing factor shows this situation as the most dominant factor in the code mixing, followed by the talking factor of speaker and partner.