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The phenomenon of natural circulation is used as a mechanism of passive cooling in nuclear reactors. This mechanism will help to dispose of residual heat in nuclear reactors when emergency conditions for example due to pump failure. Testing natural circulation systems can be done with experiments or closed-loop system simulations. Previous studies have conducted experiments with closed loop systems with variations in temperature differences between heaters and coolers. Water is used as a fluid in this experiment. The experimental results show that the temperature difference between the heater and the cooler influences to the velocity of fluid flow. For a maximum temperature difference of 80 oC, the flow velocity reaches 3 cm/s. For single phase closed loop systems with water as the fluid used, this value is no longer possible to be enlarged. In this study, a single phase closed loop system will be further investigated using computational fluid dynamics (CFD). Tests are carried out for several types of fluids to see the effect of fluid density on temperature differences in heating and cooling. Simulation results will show the temperature distribution and velocity of fluid flow in a closed loop system.