High-density ZnO nanowires doped with 4 atom % sulfur (S) and pure ZnO nanowires were grown vertically aligned on a silicon substrate. They were synthesized via chemical vapor deposition of a Zn or Zn/S powder mixture at 500 degreesC. The S-doped ZnO nanowires usually form bundles. The average diameter of the S-doped ZnO nanowires and ZnO nanowires is 20 and 50 nm, respectively. They consist of single-crystalline wurtzite ZnO crystals with a uniform growth direction of [001]. Elemental mapping reveals that the S doping takes place mainly at the surface of the nanowires with a thickness of a few nanometers. X-ray diffraction data suggest that the incorporation of S would expand the lattice constants of ZnO. The photoluminescence and cathodoluminescence of S-doped ZnO nanowires exhibit a significantly enhanced green emission band that comes from the S-doped surface region of the nanowires.