In this study, pure polyacrylonitrile (PAN) nano fiber (8 wt. %) and PAN nano fiber with multi-walled carbon nanotubes (MWCNT) were prepared. The content of MWCNT was (1, 3 and 5 wt.%). Electrospin device was used in nanofiber production. Nano diameters of the produced nanofibers were examined by Scanning electron microscopy (SEM). The nanofiber samples were cut 0.5 cm wide and 2 cm long. These samples were subjected to a tensile test on the Shimadzu device. The effect of nanofiber diameters on mechanical properties was investigated. All the MWCNT based composite nanofibers presented larger diameters than those of pure PAN nanofibers. The maximum stress value (76.60 kN) was found to be at PAN nanofiber with 1% MWCNT (average nano fiber diameter is 369.45 nm).