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This study analyzed the composition of residual gases inside a vacuum-sealed panel, which was composed of a CNT emitter cathode plate, a phosphor anode, and glass spacers, all kept in vacuum, revealing that CO, CO2, N2, CH4, H2, C2H6, and Ar existed inside the panel. The CNT emitter cathode was prepared in a diode structure by using the CNT paste, producing an array of dots with a diameter of 10 mum and a pitch of 300 mum over an active area of 2x2 cm. Prior to investigating the effect of each residual gas on the emission properties of CNTs, the sample was electrically conditioned in ozone of 5x10-4 torr to improve the emission uniformity over an area by reducing the height difference between CNT emitters. The conditioning was continued until the electric field reached 9 V/mum by applying the pulse voltages with the duty ratio of 1/60 and the frequency of 500 Hz while keeping the constant emission current of 28 muA. The chamber was then evacuated and maintained at a high vacuum of ~10-8 torr for 10 h while emitting 28 muA of electrons all the way long. Thereafter, each gas species was introduced to a vacuum chamber up to 5x10-5 torr for 2 h while continuing electron emission, and then gas injection was terminated to keep a high vacuum of ~10-8 torr for a 20 h. Here, we observed the variation of voltages under the electron emission of the constant current during these whole procedures from ozone electrical conditioning, through the injection of each residual gas, to maintaining a high vacuum. When a residual gas was introduced while emitting electrons, gas molecules would adsorb onto and then may react with CNTs. Upon gas adsorption, the voltage either increased or decreased according to an effect of gas species on work function. The opposite behavior was observed upon gas desorption. At the moment of gas exposure to O2, the voltage increased, whic- h implies an increase of work function. Upon desorption, the voltage decreased but was still higher than the initial level, meaning that O2 does not only increase the work function but also damages CNTs by oxidation. This study will discuss the effect of other residual gases on the emission characteristics of CNTs in detail.