The thixotropy model developed in this study is based on the phenomenological scheme proposed by Cheng and Evans, Brit. J. Appl. Phys., 16 (1965) 1599-1617. The idea of this model is based on the assumption that the thixotropic behavior of dispersed systems is well described by the coagulation process of the minimum sized particles contained in a dispersion system and the break-up process of coagulated clusters. Combined with the modified-cell model, the proposed thixotropy model can predict the complex theological behavior of coal-water mixtures. This model can be applied to predict the viscosity of highly concentrated solid-liquid dispersion systems which have a wide particle-size distribution.