“Nanofluids” (nanoparticle suspensions in polar or non-polar solvents) have attracted great attention in recent years as novel systems for thermal fluids management. It has been shown that significant thermal conductivity enhancement can be achieved with only a small fraction of nanoparticles in base fluids. However, the mechanism for this phenomenon (much higher thermal conduction than solvents alone) remains unclear; and in some cases, controversial conclusions may result, e.g, no thermal enhancement or negative enhancement was observed for nanofluids. Nevertheless, it has been realized that the surface effect (e.g., surface charge of nanoparticles, nanoparticle aggregation and stabilization) may play a critical role in determining or affecting their thermal properties. In order to systematically study and understand the thermal behavior of nanofluids, we have been working to use surface charge, nanoparticle composition, shape, and solvent polarity as important parameters to probe their thermal conductivity. Transit hot-wire method (THW) is used as an efficient measurement technique in determining the thermal conductivity of nanofluids prepared. We also utilize electric field as an important tool to study its effects on nanoparticle stabilization and movement in the nanofluids, and correspondingly its effect on thermal conductivity of nanofluids. The results of this study may provide new insights in the mechanism of thermal conductivity enhancement of nanofluids.