Nanofluids are stable suspension of nanometer sized particles and exhibit extremely attractive thermal properties that make them a potential candidate for application in heat transfer devices ranging from microelectronic gadgets to thermal power plants. In the present study, we have synthesized Al-5wt%Zn nanoparticles by mechanical alloying, characterized these nanoparticles using X-ray diffraction and scanning and transmission electron microscopy. Subsequently, these nanoparticles are dispersed to the tune of 0.01-0.10 vol% in ethylene glycol (base fluid) following a careful mixing protocol. Thermal conductivity of the nanofluids and base fluid has been measured using the transient hot-wire method. It is observed that thermal conductivity of the nanofluids strongly depend on the concentration, particle size, fluid temperature and stability of dispersed nanoparticles in the base fluid. A maximum of 16% enhancement in thermal conductivity has been recorded at a nanoparticle loading of 0.10 vol%. Unlike data reported in some articles, thermal conductivity ratio of Al-5wt%Zn dispersed ethylene glycol based nanofluids is observed to decrease with the increase in crystallite/grain size of the particles. (C) 2011 Elsevier Ltd. All rights reserved.