Degradation of carbon nanotubes (CNTs) significantly affects CNT's electron emission capability and limits their applications as electron emitters. In this study, CNTs were placed in controlled environment and their degradations were studied. Two types of CNT degradations, thermal and chemical degradations were identified and their effects on electron emission were studied. It was found that CNTs placed in low vacuum environment at high temperature were subject to oxidation that led to severe degradation of CNTs and their electron emission ability, whereas, CNTs treated at the same high temperatures in an ultrahigh vacuum environment only suffered from morphological changes that had minor impact on electron emission. The CNT samples used in this study were grown on tungsten substrates via plasma enhanced chemical vapor deposition (PE-CVD). The surface morphology changes of the CNT emitters were examined using scanning electron microscopy (SEM). The field emission properties of the CNTs were measured and correlated to the morphology changes.