This study aims to find out the kind of code mixing and its functions of code mixing in CriticalEleven novel. This research uses the descriptive qualitative method. The researcher chooses Critical Eleven novel as the subject of this study because the characters of this novel are bilingual. The researcher collected the data based on words, phrases, sentences, and paragraphs related to code mixing. The result after analyzing the novel, the researcher concluded that the characters in Critical Eleven novel used three types of code mixing there are insertion, alternation, and congruent lexicalization. The type that was used in Critical Eleven novel was the insertion which is 33 utterances, alternation 12 utterances, and congruent lexicalization 11 utterances. While the function of code mixing, the researchers also found 7 functions of code mixing, there was talking about the particular topic 27 utterances, quoting somebody else 1 utterance, being emphatic 2 utterances, interjection 6 utterances, repetition 3 utterances, the intention of clarifying the speech content 4 utterances and expressing group identity 13 utterances.