In this paper, we investigate and report the effects of a cationic polyelectrolyte, polydiallydimethylammomum chloride (PDADMAC), on the stability of nano-sized alumina (alpha-Al2O3) suspension. Due to the static electrical repulsion interactions, PDADMAC show significant adsorption on alpha-Al2O3 only at alkaline pH. Considerable amount of non-adsorbed free PDADMAC exist in alpha-Al2O3 suspensions dispersed by PDADMAC at acidic and neutral pH. At 1.0 wt.% PDADMAC addition, alpha-Al2O3 became highly positively charged (>50 mV) in the entire pH range contrast with that the stability of alpha-Al2O3 suspension is enhanced only over the basic pH range in the presence of anionic polyelectrolyte. Free PDADMAC electrolyte in suspension results in increasing viscosity of alpha-Al2O3 suspensions dispersed with PDADMAC at alkaline and neutral pH. However, non-adsorbed free PDADMAC in suspension do not contribute to viscosity increase in acidic alpha-Al2O3 suspensions and alpha-Al2O3 suspensions dispersed with PDADMAC show the best flow behavior at acidic pH. (C) 2006 Elsevier Ltd. All rights reserved.