In this study, Copper-Polyaniline nanocomposite (Cu-PAni) has been synthesized by gamma radiolysis as a novel method and was confirmed by different analytical techniques such as Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), X-ray powder diffraction (XRD). The copper nanoparticle completely covered by a layer of polyaniline with different thickness. The particle mean size was determined from XRD by using the Debye-Scherrer’s formula and the particle size was found to be 21 nm. The thermal conductivity of the nanofluid has been measured at different nanofluids volume fractions of 0.2%, 0.4%, 0.6%, 0.8% and 1.0%. The effect of nanofluid temperature on the nanofluid properties is measured in the range from 10 °C to 90 °C. The thermal conductivity ratio of the Copper-Polyaniline nanocomposites water nanofluid increases 107% at nanofluid temperature of 10 °C and 159% at 90 °C for a volume fraction of 1.0%. The Cu-PAni nanocomposite is a potential material for thermal conductivity enhancement of water based nanofluid.