The effect of high-pressure on the rheological characteristics of alumina-silicone oil nanofluids at room temperature is investigated in this paper. Nanofluids are engineered colloidal suspensions of nano-sized particles dispersed in a basefluid. The rheological characteristics of nanofluids are studied with pressures up to 100 MPa and particle concentrations up to 8% by mass using a high-pressure viscometer. The addition of nanoparticles is observed to increase the apparent viscosity of the nanofluids. A non-Newtonian, shear-thinning behavior is observed for both the basefluid and the nanofluids at all pressures at the higher shear rates. The critical shear rate at which shear thinning occurs is observed to be strongly influenced by particle addition. An increase in the pressure is observed to have an equal effect on the basefluid and on the nanofluids, which results in an average pressure viscosity coefficient for both to be approximately 0.015 mm(2)/N. The relative increase in the viscosity of the nanofluid to that of the basefluid was not affected by the pressure and was several orders higher than that predicted by the classical effective medium theory. (C) 2016 Elsevier B.V. All rights reserved.