Coal flotation depends on many various parameters and grain size is one of the important factors. Larger the grain size, higher is probability of its collision with air bubble and formation of the bubble-grain aggregate. Stability of the bubble-grain aggregate depends on a magnitude of the forces tending to destroy the aggregate and is a function of the grain density and size. The paper presents results of investigations of influence of the grain size on process of coal flotation. To get rid of the effect of density the grain classes floated were chosen in such manner that they had very similar densities, but different sizes. A clear difference was observed in flotation results of the grain classes having the same density and different sizes. In the case of the grains with density below 1,4 Mg/m3 their floatability increased with increasing size of the grains. However, for grains of density higher than 1,4 Mg/m3 the floatability was decreasing with increasing size of the grains.