[en] Highlights: • The RSG-GAS reactor was simulated using RELAP5/Mod3.4. • The credibility of RELAP5 code to predict the research reactor behavior was studied. • The code results are compared with experimental results in steady state and transient. • It showed a good agreement in steady state and most points of transient. • The deviation due to that the initial conditions of pool were not provided. - Abstract: The present work is developed within the frame of the IAEA Coordinated Research Program 1496, “Innovative methods in research reactor analysis: Benchmark against experimental data on neutronics and thermal-hydraulic computational methods and tools for operation and safety analysis of research reactors”. The objective of this work is to test the credibility of RELAP5 code in simulating the research reactor behavior during transients. Where, the code’s results were compared to the experimental measurements taken from Instrumented (thermocouple) Fuel Elements (IFE), installed at several positions in the core. The research reactor used in this study is the RSG-GAS reactor, located in the Serpong in Indonesia. Both the steady state and the transient simulation results performed using RELAP5 code for, are presented in this work. The calculated values for the coolant and clad surface temperature, at different locations in the core, showed an agreement with the experimental values with a difference less than 7% for the steady state, and with a difference less than 10% for transient conditions. However, RELAP5 was unable to predict the measured value of the coolant output temperature after the natural convection begins especially after stagnation occurred. In this case, the difference between the code results and experimental data was about 23%