Effective use of coal has been increasingly highlighted by the growing needs for energy sources. Among them low-rank coal including sub-bituminous coal and brown coal is an abundant resource, but it has not been competitive in thermal coal markets due to its low heating value and a tendency for spontaneous combustion. One solution to this problem is the fluidized method, Coal-Water Mixture (CWM) technique. This paper proposes a new rhelogical model of CWM. Several reports that have described the importance of a particle size distribution minimizes the void fraction among the coal particles in a low viscosity CWM. This model was semi-empirically derived from the concept of the average thickness of liquid layer among coal particles, and the relative viscosity of the slurry was described as a function of the void fraction and specific surface area of particles. The extension of the model to non-Newtonian fluids based on coagulation process was also discussed. The relative viscosity of CWM estimated by this model was compared with experimental data. The results were in good agreement with the experimental data when the void fraction of sample could be accurately calculated from the particle size distribution. In paticular, a sample in which the void fraction of coal particles is minimal does not always show the lowest viscosity. It became clear that in theory, the relative viscosity, of CWM is influenced not only by the void fraction but also by the specific surface area of particles.