Multiwalled carbon nanotube (MWCNT) arrays were synthesized by thermal chemical vapor deposition. To investigate the effect of O-2 plasma on the properties of the field-emission, MWCNT arrays were exposed to O-2 plasma. Field-emission properties, such as emission current and stability, were greatly improved after the O-2 plasma treatment. The O-2 plasma could remove the entangled thin MWCNTs and etch the tips of MWCNTs, leading to reduced density and sharpened tips. The analytical estimation extracted from the Fowler-Nordheim plot supports the observation of sharpening the tips of nanotubes with little change of the work function of nanotubes. (C) 2013 Elsevier B. V. All rights reserved.