The enhancement of the thermal conductivity of ethylene glycol in the presence of copper oxide (CuO) is investigated. CuO nanofluids are prepared in a two-step method. No surfactant is employed as a dispersant. The volume fraction of CuO nanoparticles suspended in ethylene glycol liquid is below 5 vol.-%. The crystalline phases of the CuO powders are measured with x-ray diffraction patterns (XRD). CuO nanoparticles are examined using scanning electron microscopy (SEM) to determine their microstructure. The thermal conductivities of the CuO suspensions are measured by a modified transient hot wire method. The viscosity was measured with a viscosity instrument. The results show that CuO nanofluids with low concentrations of nanoparticles have considerably higher thermal conductivities than the identical ethylene glycol base liquids without solid nanoparticles. The thermal conductivity ratio improvement for CuO nanofluids is approximately linear with the volume fraction of nanoparticles. For CuO nanoparticles at a volume fraction of 0.05 (5 vol-.%) thermal conductivity was enhanced by up to 22.4 %. CuO nanofluids thus have good potential for effective heat transfer applications.