A modified co-axial electrospinning process including electrolyte solution as sheath fluid for preparing high quality polymer nanofibers is investigated. A series of polyacrylonitrile (PAN) nanofibers were fabricated utilizing the modified process with sodium thiocyanate solutions in N,N-dimethylacetamide (DMAc) as sheath fluids. Field-emission scanning electron microscopy results demonstrated that the sheath sodium thiocyanate solutions had significant influence on the quality of PAN nanofibers. High quality PAN nanofibers in terms of fiber diameters and their distributions, surface morphology and structure have been successfully produced. The diameters of nanofibers (D, nm) could be manipulated simply by adjusting the concentrations of sodium thiocyanate (C, mg ml(-1)) in the sheath fluids with a scaling law of D = 324 C-0.1806. The mechanism about the influence of sodium thiocyanate solutions on the formation of PAN fibers is discussed and it is felt that co-axial electrospinning with electrolyte solution is a facile process for achieving high quality polymer nanofibers.