Arrays of multi-walled carbon nanotube (MWCNT) were bombarded with argon ions a nd studied with transmission electron microscopy and x-ray photoelectron spectr oscopy. Our preliminary results show that after 35min ion bombardment, most of the MWCNT curls and the iron catalyst particles residing at the top end of the tube were knocked out. Although no obvious change in its emission images is obse rved, its field emission current considerably decreases. During spontaneous cool ing down from an elevated temperature (480℃), its emission current decreases sh arply. Interesting finding is that the coating of phosphor powder on the tin-oxide-coated glass anode significantly affects the emission current measurement . For example, the phosphor coating reduces the measured current by over 30?tim es and increases the turnon field from 1.0 to 2.0V/μm.