This research was aimed at studying the implementation of English as a medium of instructionin Mathematics in a Junior High School. The subject of the research was seventh gradestudents of a Junior High School in Surakarta, Mathematics teacher, and principal of the school. This study uses interview, observation, and collecting as data collection techniques. For data analysis technique, this study uses data reduction, data display, and conclusion and drawing verification.

The study finds that observed school uses of English as a medium of instruction because of three reasons as follows: (1) School’s Curriculum uses English as a medium of instruction in Some Courses, (2) Preparing the Students for Competition and English Test, (3) Preparing the Students for Higher Level of Education. During the implementation of English as a medium of instruction in the class, the teacher prefer to uses written language rather than oral language because of many formulas in Mathematics which is easier to deliver to students by using written language.

Positive response from students was indicated by their motivation and confidence to learn during teaching learning activity. A communicative teaching learning activity increase their motivation and confidence. In addition, they believe that the use of English has some benefits for their next level of study and preparing them for English test.

Keywords : English as a medium of instruction, mathematics, students’ response