High precision electrospinning technique was used to obtain self-assembled carbon nano-tube (CNT) reinforced polyamide (PA) 6,6 fibers. The reinforcement factors were critically evaluated with respect to the effects of fiber diameter and inclusion of CNTs. The average fiber diameter ranged from 240 to 1400 nm and the CNT contents were 0, 1 and 2.5 wt%. A sharp increase in modulus and strength of the fibers was demonstrated when the size of the fiber was decreased below ∼500 nm, which could be attributed to ordered arrangement of crystals and the spatial confinement effect of the fibers. Also, investigation of the deformation behavior of fibers as a function of CNT content revealed that tensile fiber modulus and strength improved significantly with increase of CNTs. Addition of CNTs restricted the segmental motion of polymer chains and provided the confinement effect to the neighboring molecules.