The improved field emission stability of thin multiwalled carbon nanotube (thin-MWCNT) emitters using a tip sonication process has been investigated. The thin-MWCNTs showed short lengths and many open tips after the tip sonication treatment. The field emission properties of the thin-MWCNT emitters were investigated. Field emission stability dramatically increased as the tip sonication time increased. In particular, field emission current at an acceleration condition was quite stable and showed no degradation for over 19 h after tip sonication treatment of 30 min. Tip sonication could effectively cut CNTs short and regulate the length of CNTs. Therefore, field emission stability was significantly improved during a long period of operation because many shortened thin-MWCNTs could participate in field emission after the treatment.