Carbon nitride thin films were deposited on Si(100) substrate by microwave plasma-enhanced chemical vapor deposition (PECVD). Hexamethylenetetramine (HMTA) was used as carbon and nitrogen source while N 2 gas was used as both nitrogen source and carrier gas. The sp 3 -bonded C N structure in HMTA was considered significantly in the precursor selection. X-ray diffraction analysis indicated that the film was a mixture of crystalline α- and β-C 3 N 4 as well as graphitic-C 3 N 4 and β-Si 3 N 4 which were not easily distinguished. Raman spectroscopy also suggested the existence of α- and β-C 3 N 4 in the films. X-ray photoelectron spectroscopy study indicated the presence of sp 2 - and sp 3 -bonded C N structures in the films while sp 3 C N bonding structure predominated to the sp 2 C N bonding structure in the bulk composition of the films. N was also found to be bound to Si atoms in the films. The product was, therefore, described as CN x :Si, where x depends on the film depth, with some evidences of crystalline C 3 N 4 formation.