This article presents the results of a study of the mechanisms that account for the pseudoplastic behavior of a concentrated suspension of noncolloidal particles. Specifically, the measured flow behavior index of the suspension is correlated with the calculated Hamaker constant for interparticular van der Waals attraction. As the interparticular attraction increases, the number of aggregated particles increases. In turn, the number of aggregated particles decreases as the shear rate increases. As a result, unless the interparticular attraction is negligible, the concentrated suspension is pseudoplastic at low and moderate shear rates, but becomes Newtonian at high shear rates when the hydrodynamic force predominates. The measured viscosities both for high shear Newtonian limits of polystyrene suspensions and for the Newtonian suspensions of glass beads in silicone oil are in excellent agreement with the predicted values of the rigid sphere model for colloidal suspensions. In addition, we found that while its flow behavior is only slightly affected by particle size, the relative viscosity of the concentrated suspension is substantially affected by particle size, and correlates with the particle Peclet number.