U-tubes are nowadays widely used in large industries, especially in heat exchangers, as well as solar absorbers. Optimizing and increasing the thermal efficiency of heat exchangers reduces fossil fuel consumption and costs savings. A comparative experimental study was conducted on friction factor, and heat transfer of oil-based nanofluids flow (CuO and Fe3O4 with 0.5%, 1%, and 2% mass concentrations) inside inclined U-tube at 0 degrees, 30 degrees, 60 degrees, and 90 degrees inclination angles under constant heat flux. The nanofluids prepared in two steps method by dispersing nanoparticles in SN-500 base oil. The tests were performed in five different flow rates. The results showed that compared to the base fluid, using nanoparticles increased the friction factor and convective heat transfer coefficient. According to the results, it was found that the highest heat transfer rate belonged to 0.5% CuO and 1% Fe3O4, and the maximum increase in Nusselt number for nanofluids happened at 30 degrees inclination angles. Finally, it was found that the Fe3O4 nanofluid with 1% concentration had the highest value among all the test nanofluids samples in horizontal and inclined modes. (C) 2020 Elsevier B.V. All rights reserved.