Considerable demand for solid thin-film dielectrics with high dielectric constants for use in the fabrication of capacitors has been observed. In this study, polyimide (PI)/Al 2 O 3 composite films were prepared by incorporating different micron-sized β£-Al 2 O 3 contents into PI derived from pyromellitic dianyhydride and 4,4 -oxydianiline via ultrasonication. Chemical structure, morphology, dielectric and thermal properties were investigated by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), LCR meter and Perkin Elmer Pyris 6. FTIR spectra showed complete imidization, and all characteristic peaks of the imide groups are observed in PI and PI/Al 2 O 3 composite films. XRD patterns revealed that the PI/Al 2 O 3 composite exhibits peaks similar to those of β£-Al 2 O 3 , indicating that the crystal structure of β£-Al 2 O 3 remains unchanged and stable after being doped into the PI matrix. SEM micrographs showed uniform distribution of β£-Al 2 O 3 particles in the PI matrix. Meanwhile, the dielectric constant, dielectric loss and thermal stability of PI/Al 2 O 3 increases with the addition of β£-Al 2 O 3 content.