The aim of this research is to describe types and reasons of code switching used in English conversation program at Ma‟had Al-Jami‟ah Walisongo Islamic Boarding School Semarang in Academic Year of 2016/2017. This research used descriptive qualitative as research method. The researcher collected the data from recording and interview. The findings of this research showed that there are 148 data of code switching found during teaching and learning process. There are 84 data or (56.76%) which are classified as intra-sentential code switching, followed by 49 data or (33.11%) which are classified as inter-sentential code switching found in their speech. The last is tag switching, there are 15 data or (10.13%) which are classified as tag code switching. From interview result the researcher found that from students‟ perspective lacking of vocabulary is the most dominant reason they switched their language. Another reasons are: to make her friends understand, to convey opinion easily, forgetting the English words, thinking about grammar and not being fluent to speak in English. There are some reasons found from tutor‟s perspective namely lack of vocabulary, having difficulty and not being confident in English pronunciation, not master in English and less experience to speak English. Whereas, the reasons of the tutor switched their language are to make students understand tutor‟s explanation and to make them not feel bored. The implication of this reserach is code switching can be used as an effective strategy to teach students who have low English proficiency.