The side chain structures of polycarboxylates are of great importance for their dispersion performance. The impacts of side chain length (length of polyethyleneglycol PEG, m ranging from 8 to 23) of the polycarboxylates on their dispersion performances and adsorption behaviors were investigated systematically to clarify the governing dispersing mechanism. The study reveals the length of side chain affects main chain polymerization degree (DPw) of polycarboxylates, density of charge groups and adsorption in coal-water interface. Dispersion performances of polycarboxylates in coal-water slurry depend on their adsorption density and the adsorption layer thickness on the coal. The shorter side chain is beneficial to increase adsorption density. However, the longer side chain is good for increasing adsorption layer thickness. Specifically, the polycarboxylate with side chains of moderate length (m = 11) has the thickest adsorption layer and the highest adsorption density to result in its excellent dispersion performance. This information indicates that a balance between main chain length and side chain length should exist. So the polycarboxylate with the moderate length ratio of main chain to side chain can be designed as a highly efficient dispersant for low rank coal-water slurries. (C) 2016 Elsevier Ltd. All rights reserved.