Kegiatan belajar kelompok merupakan cara untuk meningkatkan kerja sama serta persaudaraan dalam kelas. Akan tetapi selama melakukan observasi, refleksi dan wawancara kepada mentor, serta siswa, ditemukan siswa kelas X-1 sekolah XYZ di Nias kurang dalam bekerja sama. Kekurangan tersebut di antaranya siswa tidak mengerjakan tugas yang diberikan secara bersama-sama, tidak menerima pendapat, tidak saling membantu, dan membicarakan hal lain ketika berdiskusi. Oleh karena itu, peneliti menerapkan metode STAD untuk meningkatkan kerja sama serta mengetahui tahapan yang benar dalam metode STAD untuk meningkatkan kerja sama. Metode penelitian yang digunakan adalah penelitian tindakan kelas (PTK) dalam tiga siklus dengan subjek penelitian siswa kelas X-1 sebanyak 22 orang. Pengumpulan data dilakukan dengan menggunakan instrumen berupa lembar observasi mentor dan teman sejawat, lembar angket siswa, serta jurnal refleksi. Data yang diperoleh dianalisis dengan teknik deskriptif kualititif. Berdasarkan hasil analisis, terjadi peningkatan di antaranya siswa mampu bersama-sama mengerjakan tugas 95%, menerima pendapat 100%, saling membantu 91%, dan siswa berada dalam tugas untuk mengerjakan tugas sesuai tanggung jawabnya 100%. Selain itu, tahapan metode STAD yang dilakukan adalah menyampaikan tujuan dan motivasi, pembagian kelompok, presentasi guru, belajar dalam tim, kuis, serta penghargaan prestasi tim. Peneliti menyimpulkan bahwa kerja sama siswa dapat ditingkatkan dengan penerapan metode STAD yang benar. / Group learning activity is a way to increase students’ teamwork and camaraderie in the classroom. However, during the observation, reflection and interviews with the teacher mentor and students, the researcher found that the students in the class X-1 XYZ school lack of teamwork. It was shown by the students who did not work together according to the task that given by the teacher, did not accept the opinion, did not help each other, and did talk out of the discussion. Therefore, the researcher decided to implement the STAD method in order to increase students’ teamwork, and also to know the STAD method’s steps. The research methodology was used Classroom Action Research (CAR) within three cycles. The subject of this research was 22 students of grade X-1. The data were collected by the mentor observation sheets, peer observation, students’ questionnaire, and journal reflection. The data were analyzed by the descriptive qualitative technique. The results showed that by the implementation of STAD method students were able to work the task together (95%), to accept the opinions (100%), to help each other (91%), and students were in a group to work the task that is given to their responsibilities. Besides, the implementation of STAD method’s steps that increase students’ teamwork were telling the objectives and motivation, dividing students in the group, teacher’s presentation, learning in group, quiz, and giving rewards for team’s achievement. The researcher concluded that the proper implementation of STAD method can increase students’ teamwork.