Associating polymers are hydrophilic long-chain molecules containing a small amount of hydrophobic groups, and they tend to create bonds between chains by reversible associating interactions. The effects of associating and nonassociating polymers on the rheological behavior are studied for suspensions of silica nanoparticles with diameters of 8, 11, 18, and 25 nm. For both polymers, the viscosity decreases, passes though a minimum, and then increases with increasing diameter. In suspensions of silica with diameters of 8, 11, and 18 nm, the direct bonds between particles to cause flocculation are not developed and the suspensions behave as fluids with the zero-shear viscosity. The suspensions of 25 nm silica are flocculated by the bridging mechanisms. The suspensions prepared with associating polymer show solid-like responses at low stresses. The associating polymer may adopt a chain conformation with a water-soluble backbone attached to the silica surfaces. The hydrophobes extending from the chains adsorbed onto different particles can form a micelle by association interactions. The multichain associations lead to the rigid bonds between particles, which are not broken by thermal energy. The geometry of particles brings about an important effect on the suspension structures and rheology.