To investigate the effect of the gun size on plasma and carbon nitride film deposition characteristics in RF magnetron sputtering system, we performed the RF sputtering deposition with different gun sizes and targets (50.8 mm and 76.2 mm in diameter). The result shows that there are remarkable different results between two magnetron sputtering guns: in plasma measurement result, the 50.8 mm gun made a column-like discharge from the target to substrate where the deposition takes place, but the 76.2 mm gun made a small volume discharge confined nearby the target, in FTIR spectra and hardness test result, the CNx films deposited by the 50.8 mm gun contained more sp(3) C-N bonds, and less hydrogenated bonds and better nano-hardness, compared with those by the 76.2 mm gun. This result is ascribed to the fact that the magnetic field configuration evolution, especially the null point difference, with varying gun size can influence the plasma structure and film property changes. (C) 2011 Elsevier B.V. All rights reserved.