Small modular reactor (SMR) concepts proposed in the past have compact size and a longer life reactor core than the conventional nuclear power plants. However, previous concepts based on the water-cooled technologies with a steam Rankine cycle have shown that the complete modularization is still technically challenging. This study suggests an innovative concept of a reactor directly cooled by supercritical carbon dioxide (S-CO2) with S-CO2 Brayton cycle as a power conversion system, namely KAIST MMR (Micro Modular Reactor). To enhance neutron economy and maximize the fuel volume fraction of the core, a uranium carbide fuel controlled by drum type control rods was used in the fast-spectrum MMR. The core has long life (20 years) without refueling or reshuffling as well as inherent safety features. The reactor can be used as a distributed power source, and replace outdated fossil fuel power plants for small cities. Moreover, we propose a S-CO2 Brayton cycle as a power conversion system to achieve small, high performance and lightweight module. Thanks to the compact core and power conversion system, the entire system can be contained in a single module. The target of the system is to achieve small and compact system that can be transported via ground transportation of a single module containing the core and the power conversion system altogether. In order to meet this target single module’s total weight is minimized in the order of 150 tons. The external size of a module is less than 7m in length and 4m in diameter. It produces 12MWe electricity from 36MWt reactor core. The S-CO2 Brayton cycle was optimized, and the cycle components were designed while maximizing the performance and minimizing the weight. A passive decay heat removal system utilizing shell & tube heat exchangers was also developed, and examined its performance with transient analysis code.