Code mixing is the entry of pieces of language into another language, while code switching is an event of transition from one language to another. Code mixing and code switching not only occur in oral conversations but can also occur in written conversations, for example the novel Negeri 5 Menara by Ahmad Fuadi. This study aims to describe (1) the form of conversation code mixing in Ahmad Fuadi's novel Negeri 5 Menara and the code mixing function, and (2) the form of conversation code switching in Ahmad Fuadi's novel Negeri 5 Menara and the code switching function. This research is a content analysis method with a qualitative descriptive form, which describes the form and function of code mixing and code switching in the novel Negeri 5 Menara by Ahmad Fuadi. The data collection technique used was document analysis. The instrument used in addition to the researchers themselves also uses a data card as a tool. The data analysis technique used is a flow analysis model. The data presentation techniques used are formal and informal. The result of the research was that code mixing was intertwined in four regional languages ​​and two foreign languages, while code switching was carried out in one foreign language. The results of this study indicate that code mixing is intertwined in four regional languages ​​(Sundanese, Javanese, Batak and Minang) and two foreign languages ​​(Arabic and English) while code switching is intertwined in one foreign language, namely Arabic. In the form of code mixing and code switching, there are words, phrases, clauses, baster and rephrase.