A major safety concern in operating a nuclear reactor is to prevent the fuel temperatures from reaching levels where they can lead to the release of fission products into the reactor vessel. This can be achieved by proper core cooling. The Nigeria Research Reactor-1 (NIRR-1) which is a tank-in-pool Miniature Neutron Source Reactor (MNSR) uses de-ionized water as coolant and moderator. The cooling process is by natural convection and hence the effect of temperature on its density needs to be ascertained to ensure effective circulation of the coolant. In this work, the density variation with coolant temperature of the NIRR-1 was measured. The results show that there is a variation in density of about 38 g/l between the NIRR-1 coolant temperatures of 0 to 100 oC. The equation relating the density variation with temperature has been established. There was strong correlation between the density variations of ordinary water with NIRR-1 (De-ionized water) water against temperature.