: Nanofluids have attracted considerable attentions over the last two decades, due to their special characteristics in various fields including heat and mass transfer, energy, mechanical and biomedical applications. Nanofluid is a fluid in which nanometer-sized particles were dispersed. Since the nanofluids benefit from the high heat transfer coefficient, therefore, one of the most important areas of their application is in the field of heat transfer. As it is evident, one of the important variables in heat transfer is viscosity. Viscosity determines not only the heat transfer characteristics, but also the fluid flow characteristics in ducts as well as pumping power. Viscosity of a nanofluid is affected by a variety of factors. In this project, the effects of the major factors such as ultrasonication duration, nanoparticle concentration, shear rate, temperature and base fluid on the viscosity and rheological properties of the nanofluid were experimentally investigated. The nanofluids studied in the present research, comprise two types of nanofluids: routine nanofluids (nanofluids containing only one type of nanoparticles) and hybrid nanofluids (nanofluids containing two or more different types of nanoparticles). The idea of using hybrid nanofluid, in fact is to achieve a trade-off between the desired properties of the constituent nanoparticles. Due to their various advantages and disadvantages, alumina and copper oxide nanoparticles have been used in this project. Among the advantages of alumina, good stability, chemical inertness, dispersibility in most fluids, ease of access, cost effectiveness and the benefits of copper oxide, relatively high thermal conductivity and anti-wear characteristics could be mentioned. Among the drawbacks of alumina, higher nanofluid viscosity increase and lower thermal conductivity compared with those of copper oxide nanofluids, and disadvantages of copper oxide, less stability of copper oxide nanofluids could be mentioned. The base fluid used in the present research is the mixture of water and ethylene glycol. The nanoparticles were dispersed in the base fluid by ultrasonication technique. By using uv-visible spectrophotometry it was found out that by increasing ultrasonication duration, dispersibility of nanoparticle in the base fluid enhanced but also viscosity of the nanofluid increased. By increasing the nanoparticle concentration it was observed that the viscosity of nanofluid went up. Among those tested, the alumina nanofluid showed the highest enhancement of viscosity and the copper oxide nanofluid showed the least, and the hybrid nanofluids, depending on the ratio of the composition of the nanoparticles, took the intermediate values, so that the nanofluids containing more alumina displayed more increase in viscosity. Alumina nanofluid with weight concentration of 1% showed up to 53% increase in viscosity relative to that of the base fluid. It was also shown the underestimation of the Einstein's and Batchelor's theoretical equations. By investigation of the effect of the shear rate, it was found out that all the nanofluids tested, had non-Newtonian and shear-thinning behavior. It was also observed that the nanofluids containing more alumina had the greater degree of shear-thinning. Another issue that was made clear, was that the degree of non-Newtonian behavior increases with the nanoparticle concentration, so that, for instance, in a low concentration, the copper oxide nanofluid showed a near-Newtonian behavior. The flow curve and the rheological behavior of the nanofluids were well described by the Cross's, Bingham's and Herschel-Bulkley's empirical equations. In the case of the effect of the base fluid, it was observed that, in the same conditions, the nanofluid whose base fluid has more percentage of water (relative to EG), although had a lower viscosity, displayed a higher yield stress. Viscosity of the nanofluids showed an exponential decrease as the temperature increased, which were well described by the Andrade's and White's empirical equations Keywords: nanofluid, hybrid nanofluid, rheology, viscosity, alumina, copper oxide, water and ethylene glycol mixture