Structure stability of multi-walled carbon nanotube under applied electric field has been investigated. The results show that anti-bonding between carbon atoms at the tip of nanotube resulting from the local field strength of ～30V/nm leads to structure instability. Using the electron microscope as the nanolaboratory, we have prepared amorphous carbon nanowires on the tip by incorporation of foreign hydrocarbon under the applied electric field, resulting in the nanotube/nanowire structure. Electric measurement indicates that the nanotube/nanowire structure has rectifying characteristics. Consequently, the applied electric field may have potential applications in structure decoration and device fabrication of carbon nanotubes.