Thermostable Nomex copoly(amide-imide)s with inherent viscosity of 0.72-1.31 dL/g were synthesized by reacting diacid-terminated Nomex prepolymer with various diisocyanate-terminated polyimide prepolymers. The polyimide prepolymer was prepared by using 4,4’-diphenylmethane diisocyanate to react with 3,3’,4,4’-benzophenonetetracarboxylic dianhydride, pyromellitic dianhydride, or 3,3’,4,4’-sulfonyl diphthalic anhydride using a direct one-pot method in order to improve their solubility. The copolymers, except B-1, P-1, P-2, D-1, and D-2, could be dissolved in N,N-dimethylacetamide +5% lithium chloride at room temperature or dimethyl sulfoxide at high temperature but were not soluble in N,N-dimethylformamide or pyridine. The solubility is considered to be related to their crystallinity. Those copolymers with crystalline structure displayed poor solubility. All the Nomex copoly(amide-imide)s had glass transition temperatures in the range of 223-352 degrees C and showed a 10% weight loss temperature of 438-574 degrees C in air and 441-585 degrees C in nitrogen atmosphere. The tensile strength, elongation at break, and initial modulus of polymer films ranged 63-118 MPa, 4-9% and 1.67-2.53 GPa, respectively. From the X-ray diffraction studies, copolymers of B-1, P-1, P-2, D-1, and D-2 with high content of PmIA showed a crystalline structure, but the others only displayed an amorphous morphology.