It is well known that dispersed particles in electrorheological fluid (ERF) form chain‐like clusters under an electric field. Many authors consider this cluster formation to be the origin of the electrically induced shear stress τe. If the shear rate γ̇ is increased, however, the clusters should be broken into shorter segments. Is this consistent with the experimental fact that τeis roughly independent of γ̇? To answer this question, we have studied the cluster formation under shear flow by simple theory and computer simulation, which are based on the force balance between the hydrodynamic resistance and electrostatic attraction among particles. The results show that, when γ̇ is increased, the length of clusters becomes shorter in proportion to γ̇−1/2, while the stress due to the clusters remains roughly constant. This indicates that the clusters are indeed the origin of the induced stress.