Unidirectional carbon fiber reinforced polymer (UD-CFRP) composites designed for ultimate tensile strength and stiffness along the fiber direction are vulnerable in the transverse direction. In this study, UD-CFRPs were reinforced in the transverse direction using sparsely distributed hierarchical Aramid pulp (AP) micro/nano-fibers between 2 and 8 g per square meter (gsm) per ply interface. Those AP micro/nano-fibers, a few hundred microns in length, effectively formed an ultra-thin interleaving layer around 20 μm in thickness between carbon fiber plies. Flexural properties along and perpendicular to continuous carbon fiber direction were measured under three-point-bending (3-P-B) to determine the toughening effects of AP. Flexural loads in both longitudinal and transverse directions have been enhanced by 30% and over 100% respectively from those randomly distributed AP micro/nano-fibers with diameters varied from a few hundred nanometers to 10 μm. Even slight increase in the elastic modulus of AP-toughened CFRP along the fiber direction was observed. Total fracture energy absorption, initial cracking and final fracture loads were measured and compared, indicating AP micro/nano-fibers have contributed to the high strength, high toughness and high modulus characteristics of AP-interleaved UD-CFRPs. Details on toughening mechanisms were revealed from SEM and cross-section optical microscopy examinations. Comparisons of unreinforced UD-CFRP and AP-toughened UD-CFRP showed the hierarchical AP micro/nano-fibers indeed played an important role in composite microstructure design and property optimization. The ultra-thin interleaved AP veil around 20 μm in thickness can be conveniently incorporated into pre-preg fabrication so that the extra interleaving step can be eliminated, i.e. the toughened pre-pregs can be used as normal pre-pregs in the final composite forming.