The failure of individual multiwall carbon nanotubes (CNTs) during electron field emission was investigated in situ inside the transmission electron microscope (TEM). Long time emission of a single CNT at the level of tens mu A or higher may lead to unrecoverable damage to the CNT. High-resolution TEM observations of the emission failure process shown that the failure was usually companied by structure damage or break of the CNT, and the failure or degradation of the emission characteristics of the CNT was typically initiated at the CNT/substrate contact, defect site or at the open end via the field evaporation or oxidation of the tip of the CNT.