The increasing public interest in sharia-based (non-ribawi) transaction and finance encourages the growth and development of Tumang Baitul Maal wat Tamwil (BMT) as an alternative sharia-based microfinance institution. This article is the result of research aiming to analyze the role of social capital in the development of Islamic microfinance institutions at the Tumang BMT, Boyolali. This research is a qualitative research with a case study approach. The unit of analysis of this study is that Tumang BMT stakeholders were selected by purposive sampling. The techniques of collecting data used were observation, indepth interviews and documentation. Data validation was carried out using source triangulation, and then data analysis was conducted using an interactive model. The results showed that social capital played a significant role in the growth and development of Tumang BMT’s existence in the community. The theoretical analysis used in this paper was Robert Putnam’s social capital approach. In his book “Making Democracy Work: Civic Traditions in Modern Italy (1993), Putnam defines social capital as the ’features of social organizations, such as networks, norms, and trust that facilitate coordination and co-operation for mutual benefits. The social capital owned by Tumang BMT includes norms based on Islamic principles that are embedded into members and customers. The members and the community’s trust grows through the implementation of principles of corporate good governance (GCG) trust, reflected on the professional service to the community. Reciprocity between BMT and the community could be accomplished through providing financial assistance and sponsorship to various events. Networking (networking) is created through “celengan bumbung’ system, Angkat Telepon Moro (ATM), Membangun Keluarga Utama (MKU), organizing football competitions and scholarship programs.