The Supercritical carbon dioxide (S-CO₂) cycle is considered as a promising power conversion system for numerous power applications, because it has relatively high thermal efficiency and compact component size. This paper shows the validation of GAMMA+ code, which was originally developed by Korea Atomic Energy Research Institute to analyze the gas-cooled reactors, by comparing the experimental data obtained from Autonomous Brayton Cycle (ABC) loop constructed in Korea Advanced Institute of Science and Technology (KAIST) and calculation result from the code. ABC loop is a simple recuperated closed S-CO₂ Brayton cycle constructed by the KAIST research team. ABC loop consists of a turbine-alternator-compressor (TAC), electric heater, recuperator, precooler and control valves. Previously, GAMMA+ code was already validated with the experimental data obtained from SCO₂PE and SCIEL facilities. In contrast to the ABC loop, only cooling and compression of the S-CO₂ were tested. With ABC loop, not only cooling and compression of the S-CO₂ were tested, but also the heating and recuperation were experimented. The ABC loop was modeled using GAMMA+ code and the calculation from the code is in accordance with the experimental data. Thus, GAMMA+ code can accurately simulate the S-CO₂ system in the future, for example a Micro Modular Reactor (MMR) utilizing as S-CO₂ power system.