This paper explores a hybrid- and two mono-nanofluids and, to be more specific, it deals with the effect of adding Silica (SiO2) and Graphene (G) nanoparticles and their hybrid (G (30%) + SiO2 (70%)) on viscosity of water. For all designed hybrid- and mono-nanofluids at various concentrations of 0.05-1%, viscosity was measured under the influence of temperature (25-50 degrees C). Meanwhile, for measuring the viscosity, different shear rates (12.23-122.3 s(-1)) have been applied to evaluate theological behavior of nanofluids. The viscosity of hybrid nanofluids lies between the viscosity of mono nanofluids, while G/Water mono-nanofluid samples have the highest value. Experimental results revealed non-Newtonian pseudoplastic behavior for all three types of nanofluids, this behavior is more evident in the G/Water mono-nanofluid. Moreover. G-SiO2 (30-70%)/Water hybrid nanofluid has a higher tendency to the behavior of SiO2/Water mono-nanofluid. Finally, via curve fitting method, two-variable correlations were proposed to estimate rheological behavior of G-SiO2 (30-70%)/Water hybrid nanofluid. (C) 2020 Published by Elsevier B.V.