The code mixing events that occurred in STKIP Singkawang students occurred due to different backgrounds. The purpose of this study was to describe the form of code mixing in the informal speech events of STKIP Singkawang students and to describe the factors that cause code mixing. The research method used a qualitative descriptive method. The data in this study were fragments of speech in the informal speech events of STKIP Singkawang students. Sources of data in this study were STKIP students in the informal realm. The data analysis method in this study used the translational equivalent method. The results of this study were code mixing in the form of words, phrases, clauses, repetition of words, and basters. The causal factors for code mixing were role identification and variety identification.