The role of method of synthesis on the size of flakes, dispersion stability and thermophysical properties of aqua based reduced graphene oxide (rGO) nanofluids is investigated in this study. It is prepared by two different synthesis methods, namely hydrazine reduced graphene oxide (rGO/H) and sodium borohydride reduced graphene oxide (rGO/S). The size of rGO/H and rGO/S flakes is found as ~ 0.1 and ~ 0.2 μm, respectively. A set of 10, 100 and 300 mg L−1 concentrations of rGO/H and rGO/S nanofluids are prepared using DI water, and the stability of rGO/H nanofluids is much better than rGO/S nanofluids due to the smaller size of flakes and the existence of –OH and –COOH groups. An enhancement in thermal conductivity of about 15.8% is found for 300 mg L−1 concentrations of rGO/H nanofluid at 75 °C. The viscosity of rGO/H nanofluids shows a negligible difference with DI water when the temperature is above 50 °C, whereas the viscosity increases with the concentrations of rGO/S nanofluids and always exhibits a higher value than base fluid. © 2020, Akadémiai Kiadó, Budapest, Hungary.