The writer identified the problems that the teacher faced in teaching Mathematics in English related to SBI program at SMA Khadijah Surabaya. Furthermore, the writer also identified the application of teaching Mathematics at SBI program. In the process of data collection, the writer used survey research approach by doing observation and recording. The writer observed and recorded Mathematics' teaching activity in four meetings. To complete the data, the writer also conducted an interview with the respondent. After analyzing all the data, the writer then found that the problems faced by the teacher involved several factors like the textbook, media and time allocation. Because of these problems, the teacher admitted that she could not teach Mathematics in English based on SBI program optimally. Furthermore, in applying the SBI program, the writer found that the teacher could not always teach Mathematics in English. The bilingual teacher preferred to use code switching that switched code from English to Indonesian. Then, the writer also found that the functions of teacher's code switching mostly appeared were repetitive function, affective function and topic switch.