Suspensions of copolyaniline containing ethoxy group, namely poly(aniline-co-ethoxyaniline), in silicone oil have been investigated as one of many potential candidates for dry-base electrorheological (ER) fluid systems. The copolyanilines were synthesized by a chemical oxidation of aniline and o-ethoxyaniline with various molar ratios in an acidic media, and the characteristics of these polymers were examined by using several techniques. By using FT-IR, SEM, and a particle size analyzer, we studied chemical structure, particle size, and the particle size distribution of the copolymer, respectively. A Physica rheometer equipped with a high voltage generator was also adopted to measure the rheological properties of ER fluids using the copolyaniline. Controlled shear stress mode was used for the shear rate sweep measurement, and the copolyaniline synthesized in this study was found to give a typical ER behavior, that is, shear stresses increase with increasing electric fields and volume fractions.