Penelitian ini didasarkan pada hasil observasi peneliti di kelas X-1 SMA XYZ. Berdasarkan hasil observasi tersebut, ditemukan bahwa kebanyakan siswa tidak turut aktif di dalam proses pembelajaran Biologi. Salah satu penyebabnya karena kebanyakan metode pengajaran hanya berfokus pada ceramah guru. Dengan menggunakan metode Numbered Heads Together (NHT), peneliti ingin mengidentifikasi penerapan NHT dalam meningkatkan keaktifan belajar siswa serta mengindentifikasi langkah-langkah penerapan metode NHT dalam peningkatan keaktifan belajar siswa. Penelitian ini merupakan sebuah Penelitian Tindakan Kelas (PTK) yang dilaksanakan di sekolah XYZ, sebuah sekolah Kristen Swasta di Jakarta. PTK dengan dua siklus dilaksanakan pada tanggal 03 Agustus-13 November 2015 dengan melibatkan 23 orang siswa. Hasil data dianalislisa dengan menggunakan analisis deskriptif kualitatif. Instrumen yang digunakan untuk mengumpulkan data adalah lembar observasi keaktifan belajar siswa, lembar observasi penerapan metode, lembar angket siswa serta refleksi peneliti. Hasil penelitian menunjukan bahwa penerapan metode NHT dapat meningkatkan keaktifan belajar siswa kelas X-1 SMA XYZ dengan menerapkan seluruh langkah-langkah metode NHT. / This research was based on the observation of the researcher in X-1 class. Based on that observation, it was found that many students did not seem to be active during the process of learning biology. One of the causes was because the method of teaching mostly focused on direct teaching. By using Numbered Heads Toegether (NHT) method, the researcher wants to identify the aplication of NHT in increasing students‟ learning activeness and also to identify the steps of the application of Numbered Heads Together method in increasing students‟ learning activeness. This research is a classroom action research that held in XYZ high school, a Christian school in Jakarta. This classroom action research with 2 cycles was held on August 3rd - November 13rd with 23 student. The data is analyzed using qualitative descriptive analysis. The instrument are used to collect the data is the observation form for students‟ learning activeness, the observation form for the implementation of method, student qustionnaire form and the researcher reflections. The result showed that the application of Numbered Heads Together method can increase the activeness of students in class X-1 in XYZ High School by applying all steps in NHT method.