We considered the electrorheological (ER) properties of an anhydrous suspension based on a semiconducting polymer. In order to investigate the effects of the electrical properties of polymeric particles on the ER phenomena, poly(p-phenylene) was synthesized and endowed with semiconductivity by doping with FeCl3 solutions. The dielectric behaviors of ER suspension revealed facilitated interfacial polarization for the appropriately doped suspension with intermediate conductivity of particle. These behaviors were found to be closely related with the magnitude of dynamic yield stress, which showed a maximum and then decreased with increasing conductivity of particles. Therefore, we established an optimum condition of FeCl3 doping of poly (p-phenylene) showing the maximum ER activity. Dynamic viscoelastic properties of the ER suspension were also characterized for the limited case. These experimental results give a clear relationship between the conductivity of semiconductive particles and their ER behaviors. Furthermore, effects of the interfacial polarization on yield stress behaviors were investigated using surfactant-activated ER suspensions. Over a wide range of temperature, our suspensions show good ER activity while retaining the low current density that would broaden the potential applicability of the ER fluids.