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A series of novel poly(amide-imide) (PAl) films with different amide contents were prepared from pyromellitic dianhydride and four amide-containing diamines.These PAl films exhibited excellent mechanical and thermal properties with tensile strength of 203.7-297.4 MPa and Tg above 407 ℃.The rigid backbone structures combined with strong intermolecular interactions provided PAl films with ultralow in-plane CTE values from-4.17 ppm/℃ to-0.39 ppm/℃ in the temperature range of 30-300 ℃.The correlation between thermal expansion behavior and aggregation structures of PAl film was investigated.The results suggested that hydrogen bonding interactions could be maintained even at high temperature,thus resulting in good dimension reversibility of films in multiple heating-cooling cycles.It is demonstrated that dimensional stabilities of PAl films are determined by the rigidity,orientation,and packing of molecular chains.Heat-resistant PAl films with ultralow CTE can be developed as flexible substrates by regulating backbones and aggregation structures for optoelectronic application.