Abstract: The density and dynamic properties of epoxy composites filled by Al₂O₃,CaF₂ and glass microbead were investigated.It was found that a dense polymeric interface layer would be formed on the surface of solid fillers due to the interaction between particulate filler surface and epoxy matrix.The properties of interface layer are different from that of cuatrix. These composites are considered as three-element systems,consisting of filler, interface and bulk resin. From concept of "Critical concentration of the filler",the density, thickness and volume fraction of interface layer are measured.