By using electrospinning technique, beaded, cylinder shaped and ribbon like ultra-fine silk fibroin (SF) fibers were obtained from its concentrated aqueous solution under different processing conditions. These fibers had an average diameter of 700 nm and were characterized by scanning electron microscopy (SEM), Wide-angle X-ray diffraction (WAXD) and Raman spectroscopy. It was found that the morphology and the secondary structure of the as-spun SF fibers were strongly influenced by the solution concentration, as well as the processing voltage. By adjusting these technological parameters, beta-sheet conformation and silk II crystal structure of the SF fibers can be formed without any post-treatment. (c) 2005 Springer Science+ Business Media, Inc.