Code switching and code mixing are the strategy of using two languages in a speech. People tend to switch or mix their language in certain occasion. The objective of this research is to find the reason why the characters switch or mix their codes. Revealing and classifying the kinds of code switching and code mixing in My Stupid Boss novel are also the aim of this study. The writer chooses novel as the object of this study because the characters of this novel are bilingual who are able to speak more than one language. The type of this research is library research. Therefore, in conducting this thesis, descriptive qualitative is used as the method of the analysis. The writer employs himself to collect data by reading the novel, and marking them to make it easier to analyze. The result of this research points out that Extern Code Mixing is frequently used by the character in the novel with 30 occurrences. Meanwhile, Intern Code Mixing is the least used with only 1 occurrence. The lack of topic-related vocabulary becomes the main reason engaging in code switching and code mixing. They tend to choose lexical items that are appropriate from the language where their repertoire is more comprehensive, that is, where the available register contains the lexemes they need in order to express themselves effectively.