A single multi-walled carbon nanotube (MWCNT)was assembled onto a W tip and transferred to an ultrahigh vacuum field-emission/field-ion microscope (UHV-FEM/FIM)for the study of field evaporation and field emission. The results showed that the field evaporation lowered the work function of the MWCNT and thus enhanced its field emission. The evaporation field of the MWCNT was estimated to be lower than 1.3×108V·cm-1 and the evaporation rate under this field was measured to be 9.4nm·min-1. The physical origin of the fact that the evaporation field of the MWCNT was much lower than the theoretical value for carbon is also qualitatively explained. Firstly，the clean end of the MWCNT，which resulted from field desorption，contained a large number of dangling bonds and the C atoms on it had small coordination number，thus the heat of sublimation of these C atoms was low. Secondly，H atoms that possibly existed in the MWCNT could collide with the C atoms under strong electric field and make the latter evaporate easier. These results indicated an effective approach to obtaining better field emission performance of a carbon nanotube by cutting it short using field evaporation.