Abstract: A series of nano Al₂O₃/polyimide (Al₂O₃/PI) composite films with different mass fraction of nano Al₂O₃ particles were successfully prepared via in situ polymerisation and thermal imidization method. The microstructure, dielectric property and mechanical property of nano Al₂O₃/PI composite films were characterized and measured by SEM, TEM, XRD, FTIR, LCR meter, high voltage power and the electronic universal material testing machine. The results show that nano Al₂O₃ particles disperse evenly in the PI matrix. The breakdown strength and tensile strength of nano Al₂O₃/PI composite films both reach a maximum value with nano Al₂O₃ particles mass fraction of 8%. The dielectric constants of nano Al₂O₃/PI composite films increase with the increasing of mass fraction for nano Al₂O₃.