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Polyimides (PI) are widely used in microelectronic industries because of their outstanding characteristics such as good mechanical, thermal and dielectric properties. In this paper, a novel PI/SiO2-Al2O3 co-doped composite film was prepared. The inorganic phases of SiO2 were added to PAA solution by sol-gel method with TEOS as precursor, while the Al2O3 phase was added as particles by ultrasonic-mechanical method. The structure and morphology of polyimide/SiO2-Al2O3 composite films were characterized by FTIR and SEM. FTIR results indicated that after SiO2 and Al2O3 addition, the Si-O-Si band drafted from 1100cm-1 to 1154cm-1, possibly indicated the new structure formation. Moreover, all the mechanical properties of PI/SiO2-Al2O3 co-doped films are higher than that of the pure PI. When the Si/Al weight ratio was 4:1 and total inorganic phases content reached 2wt.%, the electric breakdown strength of PI/SiO2-Al2O3 composite films was the highest among the samples and reached 204kV/mm, which was 24% higher than pure PI.