This study aims to improve students' critical thinking skills in social science subjects in fifth grade students of SDN Majalengka Kulon I through the cooperative learning model type of group investigation. This study uses an action research method using the Kemmis and Mc.Taggart model which consists of four stages, namely, planning, action, observation and reflection which is carried out for two cycles. Data collection techniques using field notes, documentation, observation of teacher activities and observation of student activities, tests and non-tests. The subjects in this study were fifth grade students of SDN Majalengka Kulon I. The results showed an increase in critical thinking skills. This is evidenced by the average percentage of students in the first cycle of action that is 69.9%. Teacher activities according to the steps of this method reached 67.5% and student activities reached 65%. An increase in the second cycle reached 88% of teacher activity according to the steps of this method reached 90% and student activity reached 87.5%. The implication of this research is a group investigation cooperative learning model to improve critical thinking skills. In the teaching material theme 5 is proud as Indonesia. The results of this study indicate an increase in students' critical thinking skills through the cooperative learning model group investigation type. Keywords: Critical Thinking Ability;Model Group Investigation ; Action Research.