The nanoparticles of ZnO have been dispersed in base fluid of poly(ethylene glycol), PEG. Stability of this nanofluid has been verified with UV-vis spectroscopy. Dynamic light scattering was used to obtain particle size of investigated nanofluid. Rheological behavior of this nanofluid has been investigated over a range of volumetric solid concentrations (phi(1) = 0.41-4.71%) and shear rates (gamma = 0.01-1000 s(-1)) at 298.15 K. The nanoparticle suspensions generally exhibited a pseudoplastic flow behavior, indicating an existence of particle aggregations in the liquid medium. Bingham plastic and Herschel-Bulkley models were used to evaluate the shear stress-shear rate dependency. The colloid yield stresses were determined from these models. The suspension structure was estimated from the values of fractal dimension which obtained from colloid yield stress and concentration dependency. Carreau-Yasuda model was used to evaluate the shear rate dependency of viscosity at each concentration. The new method was also proposed for correlating the viscosity values of ZnO-PEG nanofluid including concentration and shear rate dependency on the basis of Carreau-Yasuda model, Eyring absolute rate theory, NRTL, mNRF and Krieger-Dougherty models. (C) 2013 Elsevier B.V. All rights reserved.