Spherical ZnO nanoparticles each with a uniform size are synthesized by a hydrothermal method. These ZnO nanoparticles are then dispersed into water by ultrasonic vibrating to form stable nanofluids. The electrical conductivity of water-based ZnO nanofluids with a variety of temperature and volumetric fractions are investigated. The volumetric-fraction-dependent thermal conductivity is also measured at room temperature. Experiments indicate that the electrical conductivity of ZnO nanofluid reveals a non-linear relationship versus volumetric fraction. However, it presents a quasi linear relationship versus temperature. The thermal conductivity is enhanced nearly linearly with volumetric fraction increasing. Moreover, a modified model is established based on Maxwell thermal conductivity model and Brownian dynamics theory by considering boundary adsorption layer, aggregation and Brownian motion of nanoparticles in the fluid. The expected thermal conductivity values based on the modified model are in good agreement with our experimental data, suggesting that our modified model might be more accurately adapted to the nanofluids thermal conductivity.