Carbon nitride films prepared by rf magnetron sputtering were characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and grazing-incidence X-ray reflectivity. In these films, nitrogen atoms were found to be bound with sp(3), sp(2) and sp hybridized carbon and about 30% of the C-N solid may have C3N4 structure. The nitrogen to carbon ratios (NIC) of the film, sp(3) and sp(2) C-N phases were similar to0.17, 1.05 and 0.26, respectively. The density of the films was about 1.9 g cm(3). The N/C and density of the films only slightly decreased and increased, respectively, with increasing the substrate temperature. The analysis of the annealed samples indicated that these films had a good thermal stability.