Abstract: The sludge produced during the coal-to-oil process was mixed with lean coal to prepare the coal/sludge water slurries. The effects of sludge addition on the viscosity, rheological properties and stability of the slurry were investigated. Results showed that with the increase of sludge content, the viscosity of slurries decreased and the maximum slurry concentration decreased. When the sludge to coal ratio increased from 0% to 15%, the maximum slurry concentration decreased from 73.08% to 65.38%. The addition of sludge causes the coal slurry to change from expanding plastic fluid to yielding pseudoplastic fluid, and with the increase of sludge addition, the pseudo plasticity enhanced. In addition, the addition of sludge can significantly improve the stability of coal water slurry.