Micron-sized poly(urethane acrylate) (PUA)/clay nanocomposite particles were synthesized by suspension polymerization. UA containing a poly(ethylene oxide) group in the main chain was first inserted into the silicate layers of montmorillonite clay through mixing the UA with the clay. Then, ethylene glycol dimethacrylate and an oil-soluble initiator were added into this UA/clay mixture, followed by the emulsification of the monomer mixture in an aqueous solution of polymeric stabilizer. Suspension polymerization was carried out at 60degreesC for 12 h to obtain the PUA/clay composite particles. The incorporation of clay into the polymer phase was verified by FTIR spectroscopy, and the intercalation structure of the clay composite was confirmed by X-ray diffraction analysis. Other characterizations including thermal analysis, morphological observation, and dielectric analysis were also performed. After suspensions of bare PUA and PUA/clay nanocomposite particles in silicone oil were prepared, their electrorheological properties were measured under various electric fields and compared. (C) 2003 Wiley Periodicals, Inc.