Poly(3-thiophene acetic acid) was synthesized via and oxidative polymerization and blended with polyisoprene rubber (PI). Electrorheological properties of pure polyisoprene and polythiophene/polyisoprene blends were investigated for the effects of electric field strength, crosslinking ratio, and particle doncentration. Experiments were carried under the oscillatory shear mode and applied electric filed strength varying from 0 to 2 kV/mm. The dynamic moduli, G’ and G”, of the pure polyisoprene depended on the crosslinking ratio and electric filed strength; the storage modulus (G’) increased but the loss modulus (G”) decreased with increasing crosslinking ratio. The storage modulus (G’) and the loss modulus (G”) of the pure polyisoprene fluid exhibited no value change with increasing electric field strength. For PI with the crosslinking ratios of 2, 3, 5 and 7, the storage modulus sensitivity, ∆G’ increased with electric field strength and attained maximum values of 10%, 60%, 25%, and 30% at the electric field strength of 2 kV/mm, respectively. For the blends of undoped polythiophene and PI (Pth _ U/PI _ 03), with the undoped particle concentrations of 5%, 10%, 20% and 30% vol., the dynamic moduli, G’ and G” of each blend were generally higher than those of pure crosslinked polyisoprenes (PI _ 03). Their storage modulus sensitivity, ∆G’ increased with electric filed strength and attained a maximum value of 50%, 35%, 110% and 45% at the electric field strength of 2 kV/mm, respectively.