This research entitled ‘Kooperasi’ in Islamic Economic System (investigation toward Muhammad DawamRahadrjo’s Idea) is aimed to elaborate ‘Koopearis’ component and Islamic Economic System based on justice and fairness. In this research, there are three main problems: 1) What is the foundation of ‘Kooperasi’ according to Muhammad Dawam Rahardjo, 2) How is the relationship between ‘Koopeari’ and Islamic Economic System, and 3) What is the opinion of Muhammad Dawam Rahardjo for ‘Kopersi’ in Islamic Economic System. In order to address these problems, the author used literature research method and qualitative data analysis. Based on problem description and analytical method deployed in this research, the author concluded that the idea by Dawam Rahardjo for Koperasi in Islamic Economic System are: 1) Kooperasi is perceived as a syirkahta’awuniyyah, cooperation within community who share similar goals, 2) Kooperasi emphasize morality and ethic conducts in every components, and 3) Koopeasi is very relevant in the current situation, e.g., Syariah ‘Koperasi’ (BMT) is a one with full potential for future development which promotes and encourages cooperation, brotherhoods, responsibility, and justice with syariah as the foundation. In the other hand, the similarity between Koperasi and Islamic Economic System lies in the same goal to overcome the poverty and bring the prosperity for community.