The present invention discloses a process for preparing a hydrophobic fluid containing metal oxide nanoparticles dispersed therein such as heat transfer fluids or lubricant fluids, including introducing an alkaline aqueous solution (aqueous phase) and an organic solution of an organic acid metal salt (hydrophobic phase) into a rotating packed bed, the two solutions flowing radially through the rotating packed bed under a great centrifugal force, so that the aqueous phase and the hydrophobic phase contact with each other, reactants in the two phases undergo reactions at the interface of the two phases rapidly, and thus metal oxide nanoparticles are formed. The metal oxide nanoparticles stably dispersed in the hydrophobic phase, which is nanofluid.