Si nanoparticle-incorporated polyacrylonitrile (PAN) fibers are prepared using the electrospinning method and Si-filled carbon (C) fibers are obtained by the subsequent heat treatment of these Si/PAN fibers. Their microstructures are characterized by various analytical techniques. It is found that Si nanoparticles are distributed both inside and on the surface of PAN fibers and this is preserved after the formation of Si/C fibers. The crystal structure characterization indicates that, in Si/C fibers, Si nanoparticles exist in a crystalline state while carbon is in a predominantly amorphous or disordered form. Si/C fibers show high reversible capacity and good capacity retention when tested as anodes in lithium ion batteries (LIBs). The excellent electrochemical performance of these fibers can be ascribed to the combined contributions of carbon matrices and Si nanoparticles, and the favorable textures and surface properties of the Si/C fibers.