The influence of coal dust size composition on the effective viscosity of water-suspended coal fuel is analyzed. The rheological properties of the suspensions whose solid phase is a mixture of coarsely dispersed and finely dispersed coal dust fractions are studied. The mathematical models proposed by various authors to describe the dependence of the suspension viscosity on the volume content of the solid phase are scrutinized. The calculations of the effective viscosity of the water-suspended coal fuel made by using familiar models are reported. The experimental setup for determining the viscosity of the water-suspended coal fuel containing 60 wt.% of T-grade Kuznetsk coal, of which the finely dispersed fraction comprises 0-10%, is shown schematically. The water-suspended coal fuel viscosity is shown to be dependent on the content of finely dispersed dust. The mechanism of the influence of the finely dispersed coal dust fraction on the rheological properties of the water-suspended coal fuel is studied.