The utilization of oily sludge produced from refinery processes was evaluated in this study. Oily sludge was added to coal-water slurry (CWS) in different ways to prepare desirable coal-oily-sludge slurry (COSS). The slurryability and rheological characteristics of COSS were investigated and compared to CWS. The viscosity and yield stress of COSS sharply decrease with increasing the addition of oily sludge that preferentially wets coal particles. Microscopic structure and surface functional groups examination proved that oil droplets present in oily sludge could form a thin hydrophobic membrane on coal particle surface and modify its surface properties. The maximum solids loading can be either increased or decreased depending on the slurry preparation method. The maximum solids loading of COSS increases from 62.2 wt% to 64.0 wt% when oily sludge that preferentially wets coal was added in a ratio of 10.0 wt%. (C) 2014 Published by Elsevier Ltd.