This study aimed to improve students' math problem solving skills through the application of Group Investigation (GI) learning model. The subjects of this study were students of grade VII B of SMP Ma'arif NU 1 Ajibarang in Academic Year of 2018/2019 with 32 students, consisting of 20 male students and 12 female students. This is a Classroom Action Research (CAR) which consists of 3 cycles with 2 meetings in each cycle. Each cycle consists of planning, implementing actions, observing, reflecting. The instrument for measuring students' math problem solving skills used evaluation tests given at the end of each cycle. Based on the test results, it shows the average score in the first cycle evaluation was 61.41 (good criteria), the average score of the second cycle evaluation test was74.94 (good criteria) and the average score in the third cycle evaluation test was 82 (very good criteria). Therefore, it can be concluded that the implementation of the Group Investigation (GI) learning model can improve the math problem solving skills of students of class VII B of SMP Ma'arif NU 1 Ajibarang.