We report an observation of strong emission of CN- ions during heating of a nitrogen ion-irradiated graphite surface. An activation energy of 5.0 +/- 0.2 eV for thermionic CN- emission is derived from its temperature dependence. From the results of thermal desorption of neutral molecules and thermionic electron emission, it is proposed that the thermally induced CN- ions may originate from the dissociative negative ionization of desorbing C2N2 molecules on the surface via a thermionic electron attachment mechanism. This mechanism is supported by the dissociative negative ionization of the desorbed C2N2 molecules in the gas phase, as observed by negative ion RGA measurement. The large negative ionization cross section Of C2N2 molecules at an electron energy of 0.8-0.9 eV, which also leads to the formation of CN- ions, is believed to be responsible for the CN- emission during heating of the nitrogen ion irradiated graphite.