<![CDATA[This is a descriptive qualitative study designed with the aim of investigating the teacher’s use of EFL English in teaching Mathematics at Fatih Bilingual School in Banda Aceh in terms of opening, main, and closing activities. This study also investigated the obstacles encountered by the teacher in teaching Mathematics using EFL English and her efforts to overcome those obstacles. The subject of this study was a female Mathematics teacher who has been teaching tenth grade Mathematics. The research subject was chosen by using a purposive sampling technique. The data were collected through three periods of classroom observations and an in-depth interview with the teacher. The data collected was analyzed qualitatively. This study revealed that the processes of teaching Mathematics in Fatih Bilingual School involved translation. The use of English and Bahasa Indonesia were not equivalent when English became dominant. For the concepts of Mathematics, the teacher explained them in both languages. The main obstacle the teacher faced was difficulty with terminology which she had to deal with first and then had to make her students understand it too. The results also indicated that the teacher had made efforts to make the hard topics simpler so that the students could get the concepts when the teacher explained them using English. The solution for those problems was that the teacher used tools of mathematics, pictures, graphs and/or video. Code switching was also an alternative used to help the bilingual activities. In conclusion, the use of EFL English in teaching Mathematics can be done with bilingual classes. The portions of English and of the mother tongue used in the teaching activities may not be the same.]]>
