Natural circulation has an important role in the safety system of a nuclear reactor. It is part of passive system, which is operated without any external prime mover. The behavior of natural circulation shall be known to be effectively incorporated in the safety system of a nuclear reactor. This paper describes the preliminary results of the study on natural circulation conducted at a vertical rectangular experimental facility FASSIP-01. The RELAP5 code is used to simulate the experiment. The goal is to study the validity of the RELAP 5 model to analyze the natural circulation in FASSIP-01. The simulation could provide a reasonable good results describing the natural circulation established in the rectangular loop of FASSIP-01. However, there are still discrepancies with the experimental results, especially with regard to mass flow rate and temperature distribution in the loop. The model needs be improved and further comprehensive data from experiments are necessitated for better validation, as well.