Berdasarkan pengamatan yang dilakukan di kelas VII A di salah satu sekolah Kristen di Medan, peneliti menemukan bahwa kondisi di dalam kelas yang terdapat siswa menunjukkan respons pasif ketika mengikuti kegiatan pembelajaran IPA Biologi. Peneliti menyadari adanya kekurangan dalam memfasilitasi siswa dengan model pembelajaran yang menyenangkan dan menarik. Peneliti masih menggunakan metode tanya jawab, diskusi, observasi di luar kelas (karyawisata), dan ceramah sehingga siswa merasa bosan dan pasif dalam mengikuti kegiatan pembelajaran. Berdasarkan kondisi kelas tersebut, peneliti memutuskan untuk menerapkan model pembelajaran kooperatif tipe Numbered Heads Together dalam meningkatkan keaktifan belajar siswa kelas VII A. Metode penelitian yang digunakan dalam penelitian ini adalah penelitian tindakan kelas (PTK) model Pelton terdiri atas lima tahapan yaitu identifikasi masalah, pengumpulan data, perencanaan tindakan, pelaksanaan tindakan dan hasil penelitian. Penelitian ini dilaksanakan dari 20 Agustus 2018- 19 Oktober 2018. Sumber data yang digunakan oleh peneliti yaitu jurnal refleksi, lembar observasi mentor, lembar wawancara mentor, angket siswa, dan lembar umpan balik mentor. Berdasarkan hasil perolehan data yang digunakan menunjukkan bahwa penerapan model pembelajaran kooperatif tipe Numbered Heads Together dapat meningkatkan keaktifan belajar siswa. Hal tersebut terbukti bahwa siswa mengalami peningkatan dari tiga indikator keaktifan belajar yang sudah ditentukan oleh peneliti. Penerapan NHT memiliki beberapa fase yaitu pembagian nomor, memberikan pertanyaan, berpikir bersama dan menjawab pertanyaan untuk meningkatkan keaktifan belajar siswa. Kemudian, dengan adanya dorongan dari guru dapat memacu semangat dan partisipasi dari siswa dalam mengikuti kegiatan pembelajaran sehingga siswa dapat menunjukkan perilaku keaktifan belajar yang diharapkan. / The observation was made in grade VII A in one of the Christian schools in Medan. The researcher found that the conditions in the class was contained students who showed a passive response when participating in Biology learning activities. The researcher realized that the researcher was not facilitating students with fun and interesting learning process. The Researcher only used the question and answer method, discussion, observation outside the classroom (field trip), and direct teaching. Because that the students felt bored and passive when followed learning activities. Based on the condition of this class, the researcher decided to apply the Numbered Heads Together type in improving the class VII A students’ learning activeness. The research method that used in this study was classroom action research (CAR) by Pelton’s model with five steps, which are issue identification, data collection, action palnning, plan activation, and outcome assessment. This research was conducted on August 20, 2018 to October 19, 2018. Data sources used by researcher were the reflection journals, mentor observation sheets, mentor interview sheets, student questionnaires, and mentor feedback sheets. Based on the results, it showed that the application of the Numbered Heads Together type of cooperative learning model can improve students’ learning activeness. It is the evident that students have an increase in three indicators of learning activeness that have been determined by the researcher. The application of NHT has several phases which are number division, giving questions, thinking together and answering questions to improve student learning activeness. Then, the teacher’s encouragement also can stimulate the students’ enthusiasm and participation in participating the learning activities so that the students can show the expected behavioral learning