The interface plays a decisive role on the performance of nano dielectric composite films, and researchers have made tremendous efforts to confirm the existence of the interface in nano dielectric composite films. However, establishing quantitative determining interface information of nano composites is very difficult. In this paper, a typical nano dielectric, PI/SiO2 composite film with content of 10% SiO2 was prepared by an in-situ method and the microstructures of PI/SiO2 and pure PI were obtained by the small-angle X-ray scattering (SAXS), which is a non-destructive testing method. The TEM results show that the average grain diameter of SiO2 particles and the thicknesses of the interfaces are about 7.5 nm and 2.2 nm, respectively. The SAXS results are confirmed by direct TEM imaging method, in which SiO2 particles adsorb the surrounding PI molecular chains, and form the interfaces. The average grain diameters of SiO2 (not including the absorbed PI) particles and the thicknesses of interfaces are about 6.8 nm and 2.6 nm respectively. From this paper, we have proved that SAXS is a new effective means for the quantitative determining of interface information of nano composites. Display omitted