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Stable electron emission from CNT emitters at required current densities is necessary to apply to the commercial devices. Generally, the current degradation during field emission has been understood as evaporation of emitter or breaking apart of tip end under intensive electrical field. In this report, we suggest one more possible mechanism that the emission current degradation may occur by increased resistance of emitters through structural deformation under field emission. The effect of current density and time on the structure of SWNT emitters was studied with TEM, low current density (100 μA/cm2 for 1 h) application gave no detectable structural deformation with comparison to pristine SWNTs. However, the bundles of SWNTs were deformed to amorphous structure starting at tip ends or defect sites when a high current density (500 μm/cm2 for more than 1h) was applied. It supports that the deformation of SWNT results from heat during emission at emitters. It is understood that the heat during emission could give a rise to destruct the crystalline structure and reconstruct the carbon bonding in SWNTs bundles. We also understood that the increase of the turn-on voltage is attributed to the increase of electrical resistance of emitters raising from structural deformation during emission process.