Nanofluids as dispersions of fine particles within industrial fluids have potential in thermal applications due to their improved thermal characteristics. On the other hand, their viscosity may be a limitation for forced convective heat transfer, since increase in viscosity increases the pump power requirement. In this study we report experimental results for alumina-water nanofluid viscosity at different temperatures, for different nanoparticle fractions and diameters. Experimental data were collected based on a Taguchi experiment design (L8). Statistical analyses via Taguchi Method were done to determine the effects of experiment characteristics on nanofluid viscosity and relative viscosity. The viscosity of nanofluids decreased sharply with temperature (20-50 degrees C); increased with nanoparticle fraction (1-3 vol%), and increased slightly with nanoparticle diameter (10 +/- 5 nm, 30 +/- 10 nm). Taguchi Analysis revealed that the importance of the parameters on nanofluid viscosity can be sorted from lower to higher sequence as temperature, nanoparticle fraction, and nanoparticle diameter; and they were all statistically significant on nanofluid viscosity. One novel conclusion is that the interaction effect of temperature and nanoparticle volumetric fraction was significant on nanofluid viscosity at alpha = 5%, thus the effect of nanoparticle fraction was different at different temperatures, and vice versa. This interaction effect appeared in the developed nanofluid viscosity equation with a novel term, the product of temperature and nanoparticle fraction. This result may be beneficial for hydrodynamic applications, where the thermal aspects and flow characteristics need to be considered simultaneously. (C) 2017 Elsevier Ltd. All rights reserved.