Abstract: In order to study the effects of sludge on particle sizes, the particle sizes distributions of coal sludge slurry (CSSs) prepared by a wet-grinding process were studied through orthogonal test. And coal water slurry (CWSs) were also prepared at the same experiment conditions. Compared the particle sizes of CSSs and CWSs, the results shown that sludge can improve the cumulative distribution of fine particles in slurries. Dry sludge was more beneficial to the production of fine particles than wet sludge. The time of wet-grinding had a greater effect on particle sizes of CSSs than dispersant dosage. Additionally, high cumulative distribution of fine particles was one of the impact factors on the increase of CSSs’ viscosity and the decrease of slurryability.