Electrospun co-polyimide BPDA (3, 3′, 4, 4′-Biphenyltetracarboxylic dianhydride)/PDA (p-Phenylenediamine)/ODA (4, 4′-oxydianiline) nanofiber reinforced flexible composites were manufactured by impregnating these high performance nanofibers with styrene-butadiene-styrene (SBS) triblock copolymer solutions. The off-axis Young's moduli and tensile strengths of the unidirectional composites were measured and evaluated. It was found that the experimental off-axis data were in close agreement with the predicted data. Subsequently, the influence of nanofiber misalignment on the performance of the unidirectional composites was investigated and it was shown that better alignment leads to higher values of Young's modulus. Rule of mixtures was for the first time applied to electrospun nanofiber reinforced composites to back-calculate the Young's modulus of individual nanofibers. A Young's modulus of 58.9 GPa was found for these co-polyimide nanofibers, similar to commercial high-performance fibers like Kevlar 29.