The purpose of this study was to find out how effective the peer tutoring program is in improving students' English mastery? The subjects in this study were English teachers of class VII and students of class VII of SMP MBS ZAM-ZAM Cilongok. The results of the study show that choosing peer tutors cannot be determined by the teacher himself based on previous learning experience. Peer tutors can be determined through tests orally by the teacher on the material that will be taught by these students. In addition, the peer tutoring program requires careful preparation. This is because sometimes they can understand the material but cannot forbid students from seeing their friends. Supporting factors in the implementation of the peer tutoring program include interaction between teachers and students, students' mastery of English is quite good, the involvement of peer tutors in study groups makes the learning atmosphere more interesting. Apart from that, the inhibiting factors include the lack of preparation of the students who become tutors because they are appointed suddenly, the ability of students to explain material is still limited.