Among the promising advanced nuclear reactors, molten salt reactor employs various types of passive heat transfer system using the single-phase natural circulation of molten salts. The unique feature of molten salt, which has high Prandtl number, gives distinct heat transfer characteristics compared to other candidate fluids. To understand the heat transfer characteristics of molten salts, the similarity technique with a new simulant fluid was introduced based on the match of Prandtl number, in the previous studies. Extended from the previous studies, the present study investigated the unique thermal-hydraulic characteristics of high Prandtl number fluid in the natural circulation through the rectangular loop. Especially, the distinct flow phenomena of high Prandtl number fluid near the wall around the heating section were analyzed through both experimental and numerical approaches. The experimental approach employed direct observation of flow pattern at the upper part of the heating section using particle image velocimetry (PIV) technique. The visualized velocity profiles and gradients gave the clear evidence of unique flow development. Furthermore, a computational fluid dynamics (CFD) simulation using the ANSYS-CFX commercial CFD code verified the unique flow pattern of high Prandtl number fluid. With the aid of theoretical development based on the boundary layer theory, the unique flow phenomena was attributed to the enhanced local natural convection induced by high Prandtl number. (C) 2018 Elsevier Ltd. All rights reserved.