The rheological behavior of a low-rank coal-water slurry (from Lochiel, South Australia) was studied as a function of solids concentration, particle size, and size distribution. The observed significant improvement in the rheological behavior with changing particle size distribution may be explained in terms of spatial rearrangement of the particles and an apparent dilution effect. With careful control of the particle size distribution, it was possible to prepare an optimum coal-water slurry which has a low viscosity but with high solids loadings.