Berdasarkan observasi yang dilakukan peneliti terhadap siswa kelas VIII SMP XYZ di Lampung, ditemukan bahwa siswa cenderung pasif dan kurang berpartisipasi di dalam pembelajaran ditunjukkan dengan kurang fokus, tidak berinisiatif bertanya, dan tidak mencatat. Selain itu, nilai matematika siswa rata�rata berada di bawah standar KKM. Oleh karena itu, peneliti perlu meningkatkan partisipasi belajar siswa melalui penerapan metode pembelajaran kooperatif tipe Numbered Heads Together. Dengan demikian, siswa dapat menyadari tanggung jawabnya dalam pembelajaran dan dapat meningkatkan hasil belajar kognitif mereka. Metode penelitian yang digunakan adalah Penelitian Tindakan Kelas (PTK) model Kemmis & Taggart yang berlangsung selama dua siklus. Subyek penelitian adalah 12 siswa kelas VIII. Instrumen penelitian yang digunakan peneliti antara lain lembar observasi guru pamong, lembar angket siswa, jurnal refleksi peneliti, lembar tes, dan dokumentasi. Data yang diperoleh peneliti dianalisis secara deskriptif. Berdasarkan hasil penelitian siklus 1 dan siklus 2, penerapan metode NHT dengan menggunakan empat langkah besar NHT dapat meningkatkan partisipasi belajar dari kategori „baik‟ menjadi „sangat baik‟ serta hasil belajar ranah kognitif siswa kelas VIII dengan rata-rata yang mencapai standar KKM sebesar 41,67% meningkat menjadi 91,67%. Dengan demikian, metode NHT merupakan metode yang tepat untuk meningkatkan partisipasi belajar dan hasil belajar ranah kognitif siswa./ Based on the research that the researcher did in eight grader XYZ junior high school Lampung, found that students were passive and lack of participation during the learning process show through students less of focus, lack of initiative to asking, and blank record of the lesson. Besides, some students‟ grades in mathematics are still below the minimum standard of criteria. Therefore, the researcher needs to increase students‟ participation in learning through implement cooperative learning method type Numbered Heads Together. From this, student will able to realize their responsibility in learning and have a good impact to their cognitive learning result. The research method that was carried out is Classroom Action Research (CAR) with Kemmis & Taggart model. Two cycles were done in this research. The subject of this research are fourteen (14) eight grader students. The instruments that were used in this research are mentor observation sheet, students‟ questionnaire, researcher journal reflection, test sheet, and documentation. The researcher used descriptive statistics techniques in analyzing the data. Based on the result of the cycle 1 and cycle 2, the implementation of NHT with the four big steps were able to increase the students‟ participation in learning; from „good‟ become „very good‟ category. The average of students‟ learning outcomes in the area of cognitive in grade VIII reach the KKM, from 41,67% increase to 91,67%. Therefore, NHT method is the right method that can be used to increase students‟ participation in the learning process and students‟ learning outcomes in the area of cognitive.