To estimate the effects of series resistance on the field emission current associated with an array of carbon nanotubes (CNTs) and nanowalls (CNWs), the model of the floating spheres or lines between anode and cathode plates was proposed. An approximate formula for the field enhancement factor was derived, showing that the series resistance of a CNT array critically affects the field emission of vertically aligned carbon nanotube cathode, and the effect of the series resistance was not the limiting factor to the emission current in the arrayed CNWs cold cathode.