Carbon-nanotube (CNT) emitters prepared by screen printing a CNT paste vary widely in height. Upon applying electric fields, field enhancement occurs strongly on long CNTs, enabling them to emit electrons in a vacuum. In this situation, electron-emitting CNTs may be Joule-heated, during which an introduction of a small amount of an O3 gas can cause the long, Joule-heated CNTs to burn out rapidly by oxidation. We observed that such O 3 electrical trimming considerably reduced not only the average height but also the height variation of CNTs. O3 trimming greatly increased both the emission uniformity of CNT emitters by several percent and their lifetime by ∼4 times because it might increase the number of CNT emitters participating in field emission even though the emission voltage had to be raised to keep the emission current constant.