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Summary form only given. Amorphous carbon films were deposited on Si substrates by using microwave plasma chemical vapor deposition (MPCVD). It is often noted that a conditioning of carbon film is required before the onset of reproducible field emission. It involves the cycling of the voltage up and down over several cycles etc, which may be accompanied by surface damage and phase changing. The exact nature of this conditioning step and the role of the damage-induced surface structure is not fully understood. In this paper, the change of surface bonding structure by cycling of the voltage up and down, and the effect of sp2/sp3 bonding ratios of the carbon films on field emission are investigated. The surface microstructure and sp2/sp3 bonding ratios were analyzed by scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Field emission measurements of the films were carried out in a vacuum chamber with a base pressure of below 5×10-5Pa. The surface bonding structure of the as-deposited film was mainly sp2 component before doing field emission measurement. After emission conditioning treatment by cycling of the voltage up and down over several cycles, the surface sp2/sp3 bonding ratio of 1.25 was got, and the I-V characteristic of the field emission was getting reproducible and stable. The current density of 0.28 mA/cm2 was obtained at electric field of 4.8 V/μm. When the electric field was increased to 6.85 V/μm, the current density was dramatically decreased to 0.067 mA/cm2, and the surface sp2/sp3 bonding ratio was decreased to 0.53. It was revealed that surface sp2 phase in amorphous carbon films may be the main factor in obtaining low turn-on field and high current density.