In this study, oxidized and sulfomethylated lignin (OSL) with the charge density of -4.4 meq/g was used as a dispersant for coal water slurry (CWS). The impact of the OSL on CWS having different particle sizes was fundamentally investigated. The size of particles greatly influenced the adsorption of OSL on coal particles and the rheology and stability of the CWS. The maximum adsorption of OSL was found to be 0.96 mg/g on coal A having the particle size of 22 mu m (coal particles with smallest size). OSL improved the wettability of coal by 5-28%. The interfacial tension between coal particle and OSL was reduced from 17 mN/m to 13 mN/m, and the zeta potential of the suspension dropped from -30 mV to -47 mV when 0.96 mg/g of OSL was adsorbed on coal A. The addition of 64 mg/L of OSL to coal A suspension (50 wt%) reduced its viscosity from 1350 to 520 mPa.s. The stability of coal particles was also improved as the destabilization index of CWS was reduced from 4 to 0.2. These results confirmed that the OSL was an effective dispersant for the coal water slurry.