Engineered colloidal suspensions of nano-sized particles (less than 100nm) dispersed in a base fluid (nanofluid), have shown potential for industrial cooling fluids due to their enhanced heat transfer characteristics. Understanding the rheological characteristics of these suspensions is vital while employing them for flow applications. The effect of temperature on the viscosity of nanofluids at atmospheric pressure is well documented in literatures; however, there are no available data for viscosity measurements of nanofluids at elevated pressure and temperature. In this work, rheological characteristics of oil based nanofluids at high pressures and temperatures, order of 100atm and 100 C, respectively, are investigated. Nanofluid is prepared by dispersing commercially available SiO2 nanoparticles (20nm) in a highly refined paraffinic mineral oil (Therm Z-32, QALCO QATAR) which has wide applications for heat exchangers in oil industry. The rheological characteristics of both the base fluid and the nanofluid are measured using a High Pressure High Temperature (HPHT) viscometer. During experimentation, viscosity values are measured at pressures varying from 10MPa to 40MPa and temperatures ranging from 25C to 170C for nanofluid with mass concentrations of 3 percent. The viscosity values of nanofluids as well as base fluid are observed to increase with the increase in pressure. From the pressure coefficient values evaluated for basefluid and nanofluid, it is evident that the effect of pressure on nanofluid and basefluid was similar with no additional effect with respect to particle loading.