Particles of non-extractable styrene-acrylonitrile (SAN) copolymer-Na+-montmorillonite clay composite were synthesized by emulsion polymerization and then used to create an electrorheological (ER) fluid by dispersing the synthesized SAN-clay composite particles in silicone oil. The structures of the composites determined by FT-IR spectroscopy and X-ray diffraction confirmed the insertion of SAN between the interlayers of clay, whose separation consequently becomes larger than in polymer-free clay. The thermal stability of the organic polymers was found to be sustained. ER properties under the influence of externally applied electric fields were also investigated. ER fluids composed of SAN-clay composite show typical ER behavior and possess "pseudo-Newtonian" behavior at high shear rates.