Amorphous carbon nitride (a-CNx) thin films were deposited by rf-reactive sputtering method using a graphite target, and after deposition the films were exposed to oxygen plasma. The effect of the oxygen plasma exposure on the morphology and chemical bonding states of the film surface has been studied. Film composition and the chemical bonding states were analyzed by X-ray photoelectron spectroscopy (XPS). Film surface was observed by atomic force microscopy (AFM). AFM observations have revealed that the as-deposited film surface is uniformly covered with particle-like features in the early stage of deposition and the surface changes to be covered with broccoli-like features with increasing the deposition time and correspondingly the surface roughness increases, while after exposure to oxygen plasma, the film surface was etched selectively and the surface roughness increases with the plasma exposure time. It should be noted that the etching behavior depends on the film deposition temperatures. XPS studies have shown that after exposure to oxygen plasma the change in the bonding states in the films prepared at 853 K is different from that in the films prepared at RT.