We have systematically studied the electron-emission characteristics of nanostructured carbon films with various amounts of nanoclusters and degrees of nanotube alignment. According to our observation, the dense packing and/or alignment of nanotubes is detrimental to the low-field emission, which we attributed to the electrostatic screening effect. The best emission was observed from the carbon films dominated by nanoclusters; the turn-on field was 1.6 V/μm and the emission-site density was considerably higher than 5×104 site/cm2. The emission from the nanocluster-dominated film was uniform and stable. Raman spectroscopy identified the nanoclusters as crystalline graphite with some structural defects. It is conceivable that the modification of the boning hybridization at the cluster surface resulted in a diamond-like density of states, and that the corresponding small electron affinity was responsible for the excellent emission the from nanoclusters.