This is a descriptive qualitative study designing with the aim at investigating the teacher’s use of English in teaching Mathematics at Fatih Bilingual School Banda Aceh in terms of opening activities, main activities, and closing activities. This study also investigates the obstacles encountered by the teacher in teaching Mathematics by using English and her effort to overcome those obstacles. The subject of this study is a Mathematics teacher who teaches ten graders at SMA Teuku Nyak Arif Fatih Bilingual School, Banda Aceh by using purposive sampling technique. The instruments used in this study are classroom observation and in depth interview. The data were collected through three times classroom observations and in-depth interview with the teacher. The data collected were analyzed qualitatively based on Hitchcock and Hughes. This study reveals that the processes of teaching Mathematics in Fatih Bilingual School involved translation. The use of English and Bahasa Indonesia were not equivalent where English became dominant. For the concept of Mathematics, the teacher explained in both languages. The obstacles teacher faced were the difficult terminologies where she had to deal with first, then making students understand too. The result also indicates that the teacher has made effort to make hard topic simpler and understandable so that students get the concept when the teacher explains it using English. The solution for those problems was the teacher used Mathematics tools, pictures, graphs, or video and she learned students’ learning behaviour. Code switching has also become an alternative to help the bilingual activity. In conclusions, the use of English in teaching Mathematics can be applied in bilingual class. The portion of English and mother tongue used in the teaching activity may not be equal.