New aromatic poly(amide-imide) films containing isoindoloquinazolinedione unit in the backbone chain were directly obtained by thermal cyclization of the prepolymer of poly(amic acid-carbonamide) type, i.e., poly(biphenylphthalic dianhydride-oxydianiline-4,4'-diamino-3'-carbamoyl-benzanilide) [poly(BPDA-ODA-DACB)]. The films, before heat treatment, exhibited the tensile strength of 30 approximate to 40 MPa and the tensile modulus of 1.3 approximate to 1.9 GPa. After heat treatment, the strength and modulus increased to 135 approximate to 150 MPa and 2.6 approximate to 2.7 GPa, respectively. The films remained transparent throughout the thermal treatment. The cyclized films were much more chemically resistant to alkali than the commercial products such as "Kapton" film and "P84" film, while thermal properties were comparable. Toe isoindoloquinazolinedione unit induced in the chain backbone on the heat treatment of the films was considered to be a major factor responsible for the superior physical properties of the films.