In the present study, the natural convection heat transfer of liquid gallium is investigated in a rectangular loop which consists of an indirect heating block test section for heat transfer rates, 1/2 '' tubes and a condenser as well as an orifice for measuring flow rate. The mass flow rate for natural convection and average Nusselt number of liquid gallium were measured within the heat flux range of 6.17 x 10(3)-5.07 x 10(4) W/m(2). The measured heat transfer rates were correlated as follows: Nu(D) = 0.745 + 0.004Ra(D)(0.033) approximate to 0.75, corresponding to laminar natural convection regime. Also, the flow rates for the natural convection of liquid gallium depending on power level were also compared by using a CFD code and MARS-LMR (Multidimensional Analysis of Reactor Safety) code. The MARS code was modified into so-called MARS-Ga applicable for gallium-cooled systems in terms of physical properties. It was found that the predictions were in good agreements of the trend with the experimental results. (C) 2013 Elsevier Ltd. All rights reserved.