Abstract: In the course of making polystyrene-barium titanate composites, due to diffusion, etc, there exist interfaces between the scattered grains and the host medium. The dielectric properties of the interfaces are different from the dielectric properties of polystyrene and barium titanate. Considering the interfaces between polystyrene and barium titanate, a new "complex model" of dielectric constant of two-phase composites is built up. Starting from the average polarization theory and Maxwell-Garnett theory, the effective dielectric constant of two-phase random composites with an interface is presented. When the volume fraction of the scatter grain is smaller than 20%, the theoretical results on dielectric properties of polystyrene-barium titanate composites with interface are in good agreement with experimental data.