ABSTRACTThere is a contradictory theory towards the use of teachers’ codeswitchingin language acquisition. Some argues that the target language issupposed to be the only language performed in teaching process. On the otherhand, other theories allow the use of various languages as a medium ofinstruction. The second theory is strongly supported by the fact that targetlanguage learning cannot be separated from native language (Margana, 2013).Highlighting this contradictory phenomenon, this study is aimed to identify theoccurrences of teachers’ code-switching while teaching English in the classroom.Especially, this study focused on the analysis of the grammatical patterns, types,factors, and functions of code-switching. Moreover, this study revealed to whatextend these code-switching affect the students’ performance.This study was a qualitative case study which produced descriptive data.The data were all of the code-switching performed by the English teachers in theform of expressions. In order to complete the source of data, supportingdocuments and some informants were involved in this study. Coping with theaims of the study, purposive sampling was used to collect the samples under theconsideration that the samplings were teaching English in the acceleratedclassroom. Observation, documentation, and interview were conducted to collectthe data. Those were interpreted descriptively. To enhance the trustworthiness ofthe data analysis, the triangulation technique was used. After being checked, thedata were analyzed based on Spradley’s Ethnographic Analysis (1979).The results pointed out that there were two grammatical patterns of codeswitchingperformed; free-morpheme, and equivalence constraints. Regarding tothe patterns, the types of code-switching were categorized as tag-switching,intersentential switching, and intrasentential switching. The results also describedthe factors initiating the teachers to switch code; participant, topic, situation,solidarity, and status. These analyses led to the functions of teachers’ codeswitchingin teaching process; quotation, interjection, message qualification,addressee specification, reiteration, and personalization versus objectification.Moreover, it was revealed that these code-switching gave a good impact on thestudents’ performance. In conclusion, the result of this study supported the secondtheory which allows the use of various languages as a medium of teaching Englishin the accelerated classroom.Keywords: accelerated classroom program, case study, code-switching, Englishteacher