This paper is concerned with the preparation and characterization of transformer oil based nanofluids with the suspensions of Al2O3 and CuO nanoparticles . As a part of experimental study the transformer oil based nanofluids for heat exchangers in transformer cooling application, preparation and characterization has been performed. The preparation of nanofluids is the first key step in experimental studies with nanofluids. One step technique and two step technique are generally used for preparing the nanofluids. This work deals with the preparation methods of (Al2O3-Transformer oil ,CuO-Transformer oil) and characterizing the transformer oil based nanofluids of different volume concentrations (0.05%, 0.1%, 0. 5%, 1.0% and 1.5%).From the results it is revealed that increasing the volume concentration resulted in increase in thermal conductivity, viscosity of the nanofluid and decrease in density and specific heat of nanofluid.