This study aims to determine whether the application of STAD cooperative learning model is effectively used in improving students learning mastery, and how the students chemistry learning mastery on thermo-chemical subject after application of STAD cooperative learning model. This research is a Classroom Action Research.The results of research proved that the application of STAD cooperative learning model effectively used in improving students' chemistry learning mastery on thermo-chemical subject. Average learning outcomes of students in the first cycle of 74.49 with the completeness level of learning results of 82.05% (not completed in a classical), after the learning cycle II of the test results obtained learning average student learning outcome of 80.38 with a mastery level of student learning outcomes of 100% (completed thoroughly in classical). Student learning activities during learning from cycle I to cycle II by applying STAD cooperative learning model has increased. Thus, the results of this study are expected teachers especially chemistry teachers do collaborative work in the PTK as a vehicle for the development of professionalism of teachers in learning and it's time to use innovative learning, such as STAD type cooperative learning model.Key words: student learning mastery, cooperative learning model type STAD, PTK