Abstract: Composite Al_2O_3-SiO_(?) ultrafine particles were prepared by co-precipitation method. The effects of the feeding ways, the initial concentration of the reactants and the drying methods on the particle morphology were studied. The physical properties and the application behaviour of the composite powders as the filler of light solidification composite resin were investigated. The experimental results showed that the Al_2O_3-SiO_2 particles prepared are spherical, ultrafine and of good monodispersity. Some properties such as transparency, glossiness and curing degree of the composite resin were comparable with those of Bayer's products.