Abstract: Low values of viscosity and shear stress of fly ash slurry demands less pumping power in thermal power plants. In this work rheological behavior of fly ash slurry is studied. Study indicates that, additives of different nature (cationic, anionic and non-ionic polymer) help reduce viscosity and shear stress of slurry. Effect of 3 concentrations of additives (0.5%, 1% and 1.5% by weight) on viscosity of 40% fly ash slurry was studied. Viscosity index ‘n’ in each case was calculated using Oswald de Waele rheological model. Comparison is done between 3 additives in terms of effect on drag properties of slurry. Effect on pH of slurry with and without additives was studied.