The influence of emitter surface morphology on emission site density (ESD) is investigated for carbon nanotube films. The authors show that the ESD varies with cathode-anode distance for rough surface emitters while it is almost invariant for smooth ones. In addition, it has been revealed that the ESD decreases with the increase in the film area, resulting in low emission current density for large-area films. The present study suggests that the high ESD is more important to achieve high emission current density and long lifetime for film emitters than the high field enhancement factor.