Nanotechnology is foreseen to change our life by bringing new horizons to current industrial applications. It could be used, for example, to improve the performances of heat exchangers--which are widely used in industrial applications--by augmenting heat transfer characteristics of the working fluids. Nanofluids, which are engineered colloidal suspensions of nano-sized particles (less than 100nm) dispersed in a base fluid, have shown potential as industrial cooling fluids due to their reported enhanced heat transfer characteristics. However, rheological characteristics of nanofluids at extreme working conditions that occur in industrial applications are not well studied. This work for the first time presents the rheological characteristics of cooling oil based nanofluids at high temperatures and pressures. In this work, nanofluids are prepared by dispersing SiO2 nanoparticles (~20nm) in a highly refined paraffinic mineral oil (Therm Z-32, QALCO) that has wide applications in industrial heat exchangers, especially in NGL plants operated by Qatar Petroleum. Three particle concentrations of 1%, 5% and 10% by weight are considered for the investigation. The high pressure and high temperature viscosity values are measured using a HPHT viscometer (METEK CHANDLER ENGG.). In the experimentation, viscosity values of the nanofluids are measured at temperatures ranging from ambient to 160oC, with pressures varying from atmospheric to 100 atmospheres. Initial observations have shown that variation in particle loading and temperature affects the viscosity of nanofluids, whereas increases in pressure had a negligible effect on nanofluids viscosity.