A new polyimide composite was developed relying on the concept of in situ polymerization of monomer reactants polyimide. High thermal conductive, low dielectric constant and dielectric loss, and thermal-stable composites were successfully demonstrated by incorporating aluminum nitride powder into the polyimide. The weight percent of aluminum nitride was up to 80%. The thermal and dielectric properties follow the classic composite theories. (C) 2003 Wiley Periodicals, Inc.