In the present study, 3-dimensional model of natural circulation loop (NCL) have been developed in design software Proe 4.0. The steady flow simulation of single phase CO2, for natural circulation loop (NCL) with end heat exchangers are analyzed in Fluid Flow (Fluent), a module in Ansys-Workbench 14.0. Results are obtained for various mass flow rates/velocities of air in control volume of cold heat exchanger, for a fixed heat flux from air in the hot heat exchanger. Results show that due to the presence of bends and strong buoyancy effects, local velocity and temperature vary in all three dimensions.