The aim of this study was to understanding of the effect of coal particle size distributions on rheology of coal-water slurries (CWS). Experiments have been carried out on the coal samples that were different in rank. Besides two different Turkish lignites (Soma and Istanbul-Agacli), a bituminous coal from Siberia (Russia) has been used. In addition to the determination of the chemical and physical properties of the coal samples, their zeta potentials were also measured. The pulps of different solids percentage composed of coal particles with d(50) sizes of 19, 35 and 50 mum were used to determine the effect of volume fraction on the viscosity of the slurry. (C) 2003 Elsevier B.V. All rights reserved.