This research aimed to improve the student’s critical thinking skill through the implementation of Group Investigation (GI) learning model. The subject of this research were 21 class VII B student’s containing of 13 male students and 8 female students. This was a Classroon Action Research (CAR) of 3 cycles with 3 meetings in each cycle. Each cycle included planning, acting, observing, and reflecting. An evaluation given at the end of each cycle was used as the instrument to measure the student’s critical thingking skill. The results showed an average score of evaluation test in cycle I which was 52,38 with 0% learning mastery, in cycle II was 65,14 with 52,38% learning mastery, and in cycle III was 73,90 with 85,71% learning mastery. It could be concluded that the implementation of Group Investigation (GI) was able to improve critical thinking skill of Class VII B students in MTs Muhammadiyah 06 Karangreja.