The main objective of the present study is to carry out experimental investigation on thermal performance of the nanofluid-based rectangular natural circulation loop (NCL). For this study, an experimental test rig is fabricated with heater as heat source, and tube in tube heat exchanger as heat sink. For the experimentation, three different nanofluids are used as working fluids. The nanometer-sized particles of silicon dioxide (SiO 2 ), copper oxide (CuO), and alumina (Al 2 O 3 ) are dispersed in distilled water to produce the nanofluids at different volume concentrations ranging from 0.5% to 1.5%. Experiments are carried out at different power inputs and different cold fluid inlet temperatures. The results indicate that NCL operating with nanofluid reaches steady-state condition quickly, when compared to water due to its increased thermal conductivity. The steady-state reaching time is reduced by 12–27% by using different nanofluids as working fluids in the loop when compared to water. The thermal performance parameters like mass flow rate, Rayleigh number, and average Nusselt number of the nanofluid-based NCL are improved by 10.95%, 16.64%, and 8.10%, respectively, when compared with water-based NCL. At a given power input, CuO–water nanofluid possess higher mass flow rate, Rayleigh number and Nusselt number than SiO 2 –water and Al 2 O 3 –water nanofluids due to better thermo-rheological properties.