Co-polyimide nanofiber based on BPDA (3, 3', 4, 4'-Biphenyltetracarboxylic dianhydride)/PDA (p-Phenylene-diamine)/ODA (4, 4'-Oxydianiline) (BPO) were produced by electrospinning followed by imidization from precursor polyamic acid (PAA) nanofiber. The aligned co-polyimide nanofiber mats possessed a modulus, strength and strain-at-break of respective 10 GPa, 1.04 GPa, and 13.5%. In comparison with previously reported co-polyimide nanofibers BPDA/BPA/ODA (BBO) with similar chemical structures, these BPO co-polyimide nanofibers can be as stiff and strong while at the same time exhibiting moderate ductility. On the other hand, the current BPO co-polyimide nanofibers exhibited a greater toughness than previously reported homo-polyimide (BPDA/PDA) nanofibers due to their relatively high strain-at-break, leading to similar levels of toughness as Kevlar fibers. A novel and efficient way to evaluate mechanical properties of aligned nanofiber bundles (similar to 30 nanofibers in a bundle) by virtue of a micro-tensile tester was also reported. Young's modulus of 38 +/- 2 GPa and tensile strength of 1550 +/- 70 MPa were found for nanofiber bundles and were significantly higher than those of aligned mats, and are among the highest reported for electrospun fibers. Further evaluation of this bundle data using Daniel's theory based on Weibull statistics resulted in a predicted tensile strength of single co-polyimide nanofibers of about 1.9 GPa. (C) 2015 Elsevier Ltd. All rights reserved.