The multi-ethnic and multi-lingual society of Islamic boarding school students ("santri") involves the occurrence of code switching. This research aims to reveal the code switching patterns by female students in daily communication in Islamic school Al-Mukmin Ngruki Islamic Boarding School and Assalaam Islamic Modern Boarding School in Indonesia, and the factors affecting the occurrence of code switching in daily basis communication in Islamic boarding school. The data are derived from the verbal interaction among the female students in Islamic school. Meanwhile, the data are populated by using the techniques of observation, recording, and interview. Further, they are analysed by the use of Hymes, (1996) ethnography method of communication. The result of this study shows that the code switching patterns includes "Intersentensial switching," "Intrasentensial switching," and "Tag switching." However, the dominant pattern of code switching used by female students is "Intersentensial switching." It is occupied in order to clarify the sender messages. In the domain of friendship, there are switches on the language of Indonesian, English, Arabic, Javanese, Sundanese, and Malay. The cause of the code switching relates to closeness between the speakers and the partners, prestige, popular terms. Also, it intends to convey the purposes and intentions of the speakers such as confirming information and maintaining cultures.