Novel silica tube/polyimide (ST/PI) composite films prepared via ultrasonic dispersion and in-situ polymerization using STs (see Figure) from self-assembly and a PI precursor with N,N-dimethylacetamide as a solvent are reported. Dielectric constants significantly lower than that of the matrix are obtained. Effective medium theory was used to calculate the dielectric constant of the constituents. The air imported by the STs plays a crucial role in reducing the dielectric constant of the composite.