Most of small modular reactors (SMR) proposed in the previous studies have compact physical size and a long life reactor core. However, existing concepts have not achieved the full modularization. This study suggests an innovative concept of a reactor cooling by using supercritical state carbon dioxide (S-CO2). Also, the authors propose the supercritical CO2 Brayton cycle as a power conversion system to achieve compact power conversion system, and to contain the reactor core, power conversion system and containment in one module. A conceptual design of SMR module was developed, which is named as KAIST Micro Modular Reactor (MMR). MMR is a transportable modular reactor which has a long life reactor core (20 years) without fuel reloading. External cylindrical size of the MMR is 6.8m in length and 4.0m in diameter. It produces 12MWe electricity from 36MWt reactor core. The supercritical CO2 Brayton cycle was optimized and the design of turbomachineries and heat exchangers were conducted. Moreover, a dry air-cooling option to reject waste heat for inland installation was proposed. A new reactor core concept of uranium nitride fuel with drum type control rods was proposed. A passive decay heat removal (DHR) system utilizing thermosiphon loop was developed as well. This conceptual study of KAIST MMR will contribute to the realization of economic and transportable SMR.