In this study, molecular dynamics simulations were performed to understand the defect structure development of polyacrylonitrile-single wall carbon nanotube (PAN-SWNT) nanocomposites. Three different models (control PAN, PAN-SWNT(5,5), and PAN-SWNT(10,10)) with a SWNT concentration of 5 wt% for the nanocomposites were tested to study under large extensional deformation to the strain of 100% to study the corresponding mechanical properties. Upon deformation, the higher stress was observed in both nanocomposite systems as compared to the control PAN, indicating effective reinforcement. The higher Young’s (4.76 ± 0.24 GPa) and bulk (4.19 ± 0.25 GPa) moduli were observed when the smaller-diameter SWNT_(5,5) was used, suggesting that SWNT_(5,5) resists stress better. The void structure formation was clearly observed in PAN-SWNT_(10,10), while the nanocomposite with smaller diameter SWNT_(5,5) did not show the development of such a defect structure. In addition, the voids at the end of SWNT_(10,10) became larger in the drawing direction with increasing deformation.