A poly(urethane-imide) diacid (PUI), a diimide-diacid with a loft structure unit, was directly synthesized from the reaction of trimetallic anhydride and isocyanate terminated polyurethane prepolymer. FT-IR and NMR were used to characterize its chemical structure. Then PUI was blended with two types of epoxy resins with different chemical structures, diglycidyl ether of bisphenol A (DGEBA) and novolac epoxy (EPN). After Curing the blends with polyfunctional aziridine CX-100, novel polyurethane/epoxy composites were obtained as transparent yellowish films. Thermal, chemical, and morphological properties of the cured composites were investigated using thermal analysis, SEM, TEM, chemical resistance, respectively. All experimental data indicated that epoxy modified PUI composites possessed higher thermal stability than that unmodified PUI, and that modified PUI had much better chemical resistance. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2628-2637, 2009