In recent years, nanotechnology has made remarkable progress. In textile, nanofiber production using electrospinning method and after that nanofiber yarn production are taken into consideration. Using various polymer kinds in electrospinning process and yarn production leads to production of special materials like nanofiber yarns, drug delivery systems, composites and smart materials. One of the important properties of nanofiber yarns is tensile and stress-relaxation. The aim of this research is production of nanofiber yarn via two-nozzle electrospinning process from Polyacrylonitrile (PAN) as a polymer and evaluation and modeling of viscoelastic tensile behavior and stress-relaxation of nanofiber yarns. First of all, factors affecting electrospinning process including voltage, feeding rate, distance between two nozzles and electrospinning distance were optimized. Secondly, after optimization of conditions in electrospinning process, the effects of polymer molecular weight, twist and rate of elongation obtained from tensile streght apparatus on stress, strain, modulus, and work of rupture were investigated. Moreover, effects of twist and rate of elongation on stress-relaxation of nanofiber yarn were carried out. Results of tensile tests showed that increase in molecular weight and twist values lead to an increase in stress and a decrease in strain values but rate of elongation variations didn’t show any significant effects on stress values. In addition, Increase in twist and rate of elongation lead to a decrease in strain values but molecular weight variations didn’t show any significant effects on strain values. It was concluded that an increase in twist values leads to an increase in modulus values but molecular weight and rate of elongation variations didn’t show any significant effects on modulus values. Finally, increase in twist, molecular weight and rate of elongation values lead to an increase in work of rupture values. Results of stress-relaxation test showed that increase in twist and molecular weight values lead to a decrease in stress-relaxation values. Generalized Maxwell model and simple Maxwell model were chosen to simulate tensile behavior and stress-relaxation of nanofiber yarn respectively. Results of simulations showed an acceptable correlation between experimental and theoretical values. So these two models are applicable for predicting tensile and stress-relaxation behavior of nanofiber yarns.