The theological behaviour of five fly ash samples (having different d(50)) collected from a thermal power plant has been investigated over a range of volumetric solids concentration (phi = 0.32-0.4945) in shear rate range of 10-200 s(-1) The slurry was highly pseudo-plastic whose behavior can be described by a non-Newtonian power law model in the range of solids concentration studied The maximum solids fraction (4)) at which the slurry viscosity approaches infinity. was determined for the fly ash samples using the relationship between inverse of relative viscosity (i.e.1/mu(r)) and volume fraction of solids 4) A model incorporating maximum solids fraction (phi(m)), power law index (n), median particle size (d(50)), co-efficient of uniformity (C-u), shear rate (gamma) has been developed to predict the viscosity The model predictions and the experimental data matched quite well for all five samples and it is expected to be quite helpful in evaluating the hydraulic parameters for design of commercial high concentration fly ash slurry pipelines. The most important variables such as volume fraction of solids, particle size, and size distribution. and their effect on the hydrodynamic forces in a non-Newtonian laminar flow regime are discussed (C) 2009 Elsevier B.V All rights reserved