In this article, the electrospun silk fibroin (SF) fibers with an average diameter of 700 nm were prepared from a concentrated aqueous solution with an electrospinning technique. The morphology, conformation, and crystalline structure of the SF fibers were characterized by scanning electron microscopy, Raman spectroscopy, and wide-angle X-ray diffraction, respectively. The structure and morphology of the fibers were strongly influenced by the solution concentration and the processing voltage. In addition, the fiber formation parameters, including spinning velocity, elongation rate, and draw ratio, were also calculated. A kind of SF fiber with a structure between an amorphous film and a natural silk was found. We suggest that the high draw ratio was not the only factor in the transformation of SF from random-coil and alpha-helix conformations to a beta-sheet conformation. (c) 2006 Wiley Periodicals, Inc.