The carbon nitride films deposited by r.f. magnetron sputtering in pure N2 discharge were annealed in vacuum up to 900 ℃. The chemical composition and bonding structure of the films were studied using x-ray photoelectron spectroscopy, Fourier Transform Infrared. The effects of thermal annealing on the bonding structure and the electron field emission characteristics of CNx films wer e investigated. It is found that the sp2 bonds and N content in CNx films are closely related to the filed emission of CNx films. The results show that thermal annealing treatment causes a great loss of N content and a larger formation of sp2 bonds in CNx films, which would influence sign ificantly the field emission properties for the CNx films. The CNx films annealed at 750 ℃ show the optimal electron emission properties. Besides, the correlation between the chemical bonding structures and electron emission properties for the CNx films was also discussed.