Different nitrogen doped amorphous carbon (CNx) films were obtained by magnetron sputtering of carbon target in argon and nitrogen atmosphere at the increasing negative bias voltages from 0 to 150 V. The films structures have experienced great change, from the novel column to nanoporous structure at the bias voltage of 0V to the porous structure at 150 V. The proposed growth process was that the CNx nuclei grew at 0V acted as the "seeds" for the growth of the nanocolumns, and ion etching effects at 150V induced the formation of nanoporous structures. Furthermore, a comparison study showed that the field emission properties of the CNx films were related with the introduction of the nitrogen atoms, the size and concentration of sp(2) C clusters and the surface roughness. The films with rougher surface have lower threshold field. (C) 2010 Elsevier B.V. All rights reserved.