Semiconducting poly(aniline-co-o-ethoxyaniline) was synthesized by chemical oxidation polymerization of aniline and o-ethoxyaniline with two different molar ratios in an acidic media. Its chemical structure and other physical propel-ties, such as conductivity, thermal stability and density of the polymer particles, were examined. Electrorheological (ER) fluids were then prepared by dispersing the copolyaniline particles in silicone oil and their rheological properties under electric field were investigated. It was found that the ER fluids using copolyanilines show different ER behaviors than that of the homopolyaniline.