Setiap siswa ingin mencapai skor yang tinggi dalam pembelajaran, hal ini menimbulkan jiwa kompetitif serta berkurangnya sifat tolong-menolong di dalam kelas. Kondisi kelas ini juga ditemukan peneliti selama proses observasi di sekolah tempat penelitian. Permasalahan yang ditemukan adalah kesenjangan hasil belajar kognitif siswa. Fakta ini dibuktikan melalui wawancara guru mentor, catatan harian peneliti dan daftar nilai siswa dimana hanya terdapat 7 siswa yang memiliki nilai kognitif tinggi dan 16 siswa yang memiliki nilai kognitif rendah pada mata pelajaran Biologi. Untuk mengatasi masalah tersebut, peneliti menggunakan metode peer tutoring untuk meningkatkan hasil belajar kognitif siswa. Peneliti menggunakan metode Penelitian Tindakan Kelas (PTK) model Kemmis dan Mc Taggart yang berlangsung selama dua siklus dengan subyek penelitian siswa kelas XI IPA Sekolah QQ Kupang dengan jumlah siswa 23 orang. Pengumpulan data dilakukan dengan menggunakan beberapa instrumen yaitu : lembar observasi mentor, lembar angket siswa, lembar wawancara mentor dan siswa, tes siswa, dan catatan harian peneliti. Hasil penelitian menunjukkan indikator hasil belajar kognitif siswa telah mencapai kriteria yang sudah ditentukan yaitu terjadi peningkatan dari 65,21% menjadi 100% dari jumlah keseluruhan siswa mencapai nilai di atas KKM (72). Berdasarkan hasil tersebut dapat disimpulkan bahwa hasil belajar kognitif siswa dapat meningkat melalui penerapan metode peer tutoring sesuai dengan strategi kunci keberhasilan penerapan. / Every student eagerly reaches a high score or high achievement in learning thus created the sense of competition and the nature of mutual assistance did not appeared in the classroom. The conditions discovered by the researcher during the process of observation in the school where the research took placed. The problem was the students' range gap of cognitive learning outcome where there were only 7 students who had high cognitive score and 16 students who had low cognitive score in Biology. This evidence found out through mentor teacher interviews, researcher’s personal journal and students’ score list. To overcome this problem, peer tutoring method was used by the researcher to improve students' cognitive learning outcomes. The researcher used Classroom Action Research (CAR) modeled by Kemmis and Taggart through two cycles. The research subjects were 23 students of grade XI Science in Kupang School. The data collection was collected by these following instruments such as: the observation sheets mentor, student questionnaire sheet, sheet mentor and student interviews, students’ test, and researcher’s personal journal. The results showed that the indicators of cognitive students’ achievement has reached a pre-determined criteria, there was an improvement from 65,21% into 100% from the total number of the students already reached the Minimum Passing Grade (≥ 72). Based on these results it could be concluded that students' cognitive learning outcomes could improve through the implementation of peer tutoring method in accordance with the key to successful strategy implementation.