This article presents the results of an experimental study on the preparation and properties of new ternary composites composed of nano-Al2O3 particles, polyester, and epoxy resin. The ternary composites were prepared by the addition of the nano-Al2O3 particles in a binary matrix, with elevated viscosity, of the epoxy resin modified by the polyester. The nano-Al2O3 particles were previously located and dispersed in the polyester phase. The study showed that the ternary system was a type of nanoscale dispersed composite with high strength and toughness as well as modulus, combined with excellent dielectric and heat-resistance properties. All related properties of the composites were remarkably superior to those of both the binary matrix and the unmodified epoxy resin. (C) 2002 John Wiley & Sons, Inc.