Carbon nanofibers containing a range of nitrogen contents of 1-10 atom % were directly synthesized by catalytic chemical vapor deposition over nickel-based catalysts at 350-600 degrees C using acetonitrile and acrylonitrile. The nitrogen content was controlled by careful choice of the reaction conditions. The N-doped carbon nanofibers showed herringbone structure with 20-60 nm diameter. X-ray photoelectron spectroscopy was applied to examine the chemical state of nitrogen in carbon nanofibers. Structural features of N-doped carbon nanofibers were examined in X-ray diffraction and electron microscopy. The mechanism for nitrogen including the structure of carbon nanofibers through the catalysis was discussed on the basis of the results.