In this paper, we report the preparation and characterization of electrospun Nylon66 composite nanofibers incorporated with treated and untreated multi-walled carbon nanotube (MWCNT). In order to improve the mechanical properties, we added surface treated MWCNT (TMWCNT) in to the Nylon66 composite nanofibers. The resultant composite nanofibers were characterized by using scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, UV-vis spectroscopy and thermogravimetric analysis. The morphological analysis revealed that very clear arrangement of ultra-fine mesh-like nanofibers strongly bound with the main fibers. The mechanical properties of the composite nanofibers such as Young's modulus, tensile strength and elongation-at-break were improved than pristine Nylon66 nanofibers. The addition of a relatively small amount (0.5 wt%) of TMWCNT in to the electrospun Nylon66 composite nanofibers significantly enhanced the mechanical properties. The improved mechanical property of the composite nanofibers can be explained based on thermal studies. © 2013 Elsevier Ltd and Techna Group S.r.l.