The role of the initial degree of ionization (alpha(i)) of polyethylenimine (PEI) in the dispersion of SiC nanopartilces in water was investigated by sedimentation and rheological measurements. The ionization of PEI was characterized by potentiometric titration, which indicated a pH-dependent conformational transition. The dispersion of SiC particles in the presence of PEI was found to strongly depend on the alpha(i). In the presence of 0.4% by weight PEI, the 23.8% SiC by volume (Phi(SiC) = 23.8) slurries showed a Newtonian behavior for alpha(i) = 0.12-0.4 values, whereas a shear-thinning behavior was observed for alpha(i) > 0.4 in the optimal pH range of around pH 4. The rheological behavior of the slurries exhibited a strong dependence on the concentration of PEI of alpha(i) = 0.12-0.4 and the slurry showed a Newtonian behavior at an optimal concentration of 0.4% by weight. The stabilization may be dominated by an electrosteric effect arising from the adsorbed PEI of alpha(i) = 0.12-0.4. The flocculation mechanism of the slurries with PEI of alpha(i) 0.4 is also discussed.