Field emission characteristics of graphite nanofibers (GNFs) are studied by operating a scanning atom probe as a field emission microscope. The Fowler-Nordheim plot indicates that the work function of GNF is larger than that of carbon nanotubes and is comparable to tungsten. Field emission patterns indicate that electrons are emitted from the edges of graphene sheets in the direction perpendicular to the GNF axis. The ratio of the field emission voltage and that of the field evaporation voltage implies that the binding between the graphene sheets is weak. The field evaporated cluster ions of carbon and hydrogen indicate that hydrogen is not distributed randomly but is forming the characteristic carbon-hydrogen cluster, C23H2. The proposed structure of the cluster is the triangularly arranged six hexagonal cells with hydrogen terminated two carbon atoms and an extra carbon atom. (C) 2008 American Vacuum Society.