A novel core-shell structure Ag@Al2O3 nano-particles were synthesized and doped into polyimide as conductive fillers to prepare the composite films with high dielectric properties and low dielectric loss. The morphology and structures of the Ag@Al2O3 nano-particles were characterized by transmission electron microscopy(TEM), X-ray diffraction(XRD), and UV-visible spectroscopy. All the results proved that the Ag@Al2O3 nano-particles had a typical core-shell structure, for the Ag particles were coated by Al2O3 shell and the average sizes of Ag@Al2O3 particles were between 30 to 150 nm. The as-prepared Ag@Al2O3 nanoparticles were doped into the polyimide with different mass fractions to fabricate the Ag@Al2O3/PI composite films via in-situ polymerization process. SEM analysis of composite films showed that the Ag@Al2O3 nanoparticles homogeneously dispersed in polyimide matrix with nanoscale. As dielectric materials for electronic packaging systems, the Ag@Al2O3/PI composites exhibited appropriate mechanical properties and enhanced dielectric properties, including greatly enhanced dielectric constant and just a slight increase in dielectric loss. These improvements were attributed to the core-shell structure of fillers and their fine dispersion in the PI matrix. A novel core-shell structure Ag @ Al2O3 nano-particles were synthesized and doped into polyimide as conductive fillers to prepare the composite films with high dielectric properties and low dielectric loss. The morphology and structures of the Ag @ Al2O3 nano-particles were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and UV-visible spectroscopy. All the results proved that the Ag @ Al2O3 nano-particles had a typical core-shell structure for the Ag particles were coated by Al2O3 shell The average sizes of Ag @ Al2O3 nanoparticles were 30 to 150 nm. The as-prepared Ag @ Al2O3 nanoparticles were doped into the polyimide with different mass fractions to fabricate the Ag @ Al2O3 / PI composite films via in-situ polymerization process. SEM analysis of composite films showed that the Ag @ Al2O3 nanoparticles homogeneously dispersed in polyimide matrix with nanoscale. As dielectric materials for electronic packaging systems, the Ag @ A1203 / PI composites versa ropriate mechanical properties and enhanced dielectric properties, including greatly enhanced dielectric constant and just a slight increase in dielectric loss. These improvements were attributed to the core-shell structure of fillers and their fine dispersion in the PI matrix.