An unsaturated polyester-organophilic montmorillonite hybrid was prepared by dispersing polymerizable quaternary ammonium-modified montmorillonite in an unsaturated polyester resin, followed by cross-linking reaction. The purpose of this investigation was to discover the role of the polymerizable group of quaternary ammonium in improving interfacial interaction between the silicate layers and polymer chains and the mechanical properties of unsaturated polyester-montmorillonite hybrids. It is found that when the content of organophilic montmorillonite is between 2 and 5%, the tensile strength, impact strength, heat resistance, and swelling resistance of the hybrid are obviously enhanced and are better than that of the composites prepared with pristine or nonpolymerizable quaternary ammonium-modified montmorillonite. Results of X-ray diffraction and transmission electron microscopy show that unsaturated polyester and styrene in the resin can be intercalated into the interlayer space of organophilic montmorillonites. (C) 2004 Wiley Periodicals, Inc.