We theoretically investigate the relations among electrorheological (ER) effects of ER fluids, zero-field dispersity of colloidal particles and particle wettability, and find that well dispersed particles resulting from particle wettability have much stronger attraction than those aggregated into different clusters. We reveal that, to achieve ER effects, besides improving dielectric properties of ER particles, it is also helpful to reduce the surface tension of liquids, which leads to higher wettability of suspended particles and thus better zero-field dispersity of the particles due to lyophilic repulsive forces between them. Our theoretical results are applied to explain qualitatively the experiments by others. (c) 2007 Elsevier B.V. All rights reserved.