Poly(p-phenylene) (PPP) particles were synthesized from a bulk polymerization of benzene and then doped with FeCl3 in aqueous solution to control their electrical conductivity for their electrorheological (ER) application. Various concentrations of the aqueous FeCl3 were implemented to investigate the effect of doping degree on ER performance, and the yield stress of an ER fluid, which is a typical characteristic of a Bingham fluid, was found to be related to both the applied electric field strength and the doping degree. A universal scaling function was introduced to collapse all of the data. From dynamic experiments, it was also found that the semiconducting PPP-based ER fluids exhibit characteristics of a viscoelastic solid, which increases with electric field strength and doping degree.