Polymer nanocomposite fibers based on polyacrylonitrile (PAN), reinforced with different morphologies of ZnS nanoparticles at different percentage loadings were prepared using the electrospinning method. The thermal decomposition profile of the nanofibers was investigated using thermogravimetric analysis (TGA). The morphology and structure of the nanofibers were studied by scanning electron microscopy (SEM) and X-ray diffraction. TGA results indicated an enhanced thermal stability of the fibers after incorporation of the nanoparticles. SEM images indicated a linear relationship between the diameter of the PAN-ZnS fibers and the concentration of the hybrid composite solution. X-ray diffraction showed that the ZnS nanoparticles were in cubic phase.