Recently, demand of distributed power source is rapidly increasing at remote region with limited access to the large source of cooling water. To respond to this demand, KAIST research team is developing a Supercritical CO2 (S-CO2) cooled small modular reactor(SMR) called KAIST Micro Modular Reactor (MMR). The S-CO2 cycle is receiving significant attention from the power industry since the cycle has high efficiency, simplicity, compact components. Moreover, SMR cooled by water could not be completely modularized including the power conversion system because of bulky components of steam cycle, but MMR could be perfectly modularized including the power conversion system due to the compactness of its components. A priority of designing MMR is small in size and mass, which can be transported via ground transportation. A conceptual design of turbomachinery, heat exchanger and reactor core of MMR are developed. However, transient analysis of MMR was not yet carried out, because a transient code for S-CO2 cycle is not readily available. Thus, KAIST research team has modified GAMMA+ code which was previously developed by KAERI research team to simulate Very High Temperature Reactor (VHTR) system. Original GAMMA+ code was not equipped with exact CO2 properties especially near the critical point. Thus, CO2 properties from NIST database was inserted in the modified GAMMA+ code first. Another modification is modeling turbomachinary for S-CO2 by using a performance map. By using this modified GAMMA+ code, control logics were tested for the safe operation of the nuclear power plant. Furthermore, loss of load and Loss of Coolant accident was analysed to demonstrate the safety of KAIST-MMR.