The critical capability of Natural circulation flow for the removing heat has been demonstrated and are widely used in the industry particularly received notable attention to improve passive safety and decrease running expenses in the latest version of nuclear reactors. Among the different type of natural circulation systems, the distinctive characteristics of single-phase natural circulation loops (NCLs), those involved even in very simplified models, are identified to be characterized by a deep complexity of chaotic behavior. In this investigation the experimental data gathered in the AKIAU-R-1P loop, a rectangular single-phase NCL through a test operation, are compared with the results of simulations obtained by one-dimensional RELAP5/Mod3.2 code and three-dimensional ANSYS FLUENT 17.1 computational fluid dynamic (CFD) code. The experimental data, including time histories of the temperature difference over the heater, was almost similar to the predicted simulation results of RELAP5 and ANSYS FLUENT. The RELAP5 code has the capability of prediction the status at low power levels (stable mode) with adequate correctness rather than CFD code whereas when the system is unstable at higher power, CFD indicates further oscillatory behavior in comparison with RELAP5 code and experimental data. The CFD code was functional in presenting the diffusive phenomena which could not be estimated by the RELAP5 one-dimensional model. (C) 2019 Elsevier Ltd. All rights reserved.