Oil-based nanofluids were prepared by dispersing copper nanoparticles into a high-temperature synthetic oil using a two-step method. The nanofluid stability was evaluated by measuring the effective thermal conductivity over time. The thermophysical properties of the nanofluids were investigated by measuring the effective thermal conductivity up to a maximum temperature of 210 degrees C using the transient hot wire method. The thermal conductivity was increased with increasing Cu nanoparticle volume fraction and decreased with temperature between 30 and 210 degrees C. A maximum thermal conductivity enhancement of about 18% was obtained at 30 degrees C with the 0.02 volume fraction of Cu nanoparticles. The experimental data below 150 degrees C compares well with predictions of the Jang and Choi model. Above 150 degrees C, the effective thermal conductivity enhancement is much lower than predicted by the model, so new mechanisms are needed to explain this phenomenon. (C) 2014 Elsevier B.V. All rights reserved.