Polyethylenimine (PEI, M-w, 50,000) was employed as a dispersant to stabilize nanosized ZrO2 (3 mol% Y2O3 doped) aqueous suspensions, Adsorption isotherm measurements demonstrated that the plateau region of coverage was reached when polymer concentration approached 1.0 wt% dry weight basis of ZrO2, and beyond at fixed pH values. The positively charged polymer chains showed a high affinity type of adsorption on nanosized ZrO2, particles, Obviously, the amount of dispersant adsorbed on ZrO, particles at pH higher than the isoelectric point was greater than the amount adsorbed at low pH. FTIR spectral analysis of the polymer-anchored ZrO2 nanoparticles strongly supported the conclusion that the adsorption was accompanied by the formation of a ZrO2/PEI interface complex. The mechanisms of PEI adsorption on ZrO2 nanoparticles were dominated by hydrogen bonding and electrostatic attraction.