Two series of sulfonated poly(styrene‐co‐divinyl benzene)s (SSDs), different in degree of sulfonation and in average particle size, were synthesized. The electrorheological behavior of the suspensions of these SSDs dispersed in silicone oil was investigated. Shear stress induced by application of an electric field increased with increase in the degree of sulfonation and also in particle size. The sedimentation velocity of these suspensions increased monotonically with the degree of sulfonation and the average particle size. Another series of sulfonated poly(styrene‐co‐divinyl benzene), which had sulfonated surface layer and unsulfonated core, (SSDH), was separately prepared. Suspensions of SSDH showed electrorheological effect similar to those of SSD. The use of SSDH improved the stability of the suspension.