A novel heterocyclic diamine monomer, 2-(4-aminophenyl)-6-amino-4(3H)-quinazolinone was synthesized from readily available compounds in three steps in high yield. A series of novel polyimides containing the quinazolinone moiety were synthesized either by a one-step solution polymerization reaction or by a two-step procedure that included a ring-opening polyaddition to give poly(amic acid)s, followed by cyclodehydration to polyimides. In the one-step method in m-cresol or p-chlorophenol with isoquinoline catalyst, polyimides with inherent viscosities of 0.44-0.63 dL/g mere obtained. Polyimides 4f and 4g from dianhydrides which have a flexible linkage, hexafluoroisopropylidene dianhydride (2f) and BPA. dianhydride (2g), were soluble in hot m-cresol and could be cast into tough films from m-cresol solution. In the two-step method, poly(amic acid)s with inherent viscosity between 0.63 and 1.40 dL/g were obtained in DMAc at roam temperature. The poly(amic acid)s were converted to the corresponding polyimide films by thermal cyclodehydration. The T-g values of the polyimides ranged from 298 to 350 degrees C. Polyimide 4a from pyromellitic dianhydride (2a) did not show a clear transition in the DSC. Polyimides 4b from biphenyltetracarboxylic dianhydride (2b) and 4g were semicrystalline and showed clear endothermic peaks for T-m at 465 and 356 degrees C, respectively. Degradation temperatures for 5% weight loss all occurred above 540 degrees C in nitrogen and above 537 degrees C in air. Young’s moduli of thin films ranged from 2.34 to 4.89 GPa at room temperature and they maintained the high moduli to high temperatures. The monomer was successufuly copolymerized with 4,4 ’-oxydianiline and 2g. With increasing incorporation of quinazolinone units, the copolymers had higher T-g values, were more thermally stable, and had higher moduli.