Co-polyimide (co-PI) fibers with outstanding mechanical properties were fabricated via thermal imidization of polyamic acids, derived from a new design of combining the amide and benzimidazole diamine monomers, 4-amino-N-(4-aminophenyl)benzamide (DABA) and 2-(4-aminophenyl)-5-aminobenzimidazole (BIA), with 3,3,4,4-biphenyltetracarboxylic dianhydride (BPDA). The crystalline structure and micromorphology of the prepared co-PI fibers were investigated by synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). The two-dimensional WAXD spectra imply that the co-PI fibers possess a structure between smectic-like and three-dimensionally ordered crystalline phase, and all the obtained fibers are highly oriented along the fiber axis. SAXS patterns exhibit a pair of meridional scattering streaks for the homo-PI (BPDA/BIA) fiber, suggesting the presence of periodic lamellar structure. The incorporation of DABA into the polymer chains destroyed the lamellar structure but led to smaller size of microvoids upon increasing DABA moiety, based on SAXS analysis. The co-PI fibers, with the molar ratio of BIA/DABA being 7/3, exhibited the optimum tensile strength and modulus of 1.96 and 108.3 GPa, respectively, attributed to the well-defined ordered and dense structure. The chemical structure and molecular packing significantly affected the thermal stability of fibers, resulting in the different glass transition temperatures (T-g) from 350 to 380 degrees C. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 183-191