Carbon nitride thin films were fabricated by plasma enhanced chemical vapor deposition (PECVD) using an Si3N4 interlayer. X-ray photoelectron spectroscopy (XPS) and transmission electron spectroscopy (TEM) were used to characterize the thin films. The nitrogen content of the thin films is up to 42.96 at.%, and C1s and N1s binding energies are 285.01, 398.60 eV respectively, as determined by XPS. The results of TEM suggest that the cubic C3N4 grains with typical dimensions of 0.4-2 mu m are embedded in an amorphous C-N polymer. The cubic C3N4 has bcc structure with a lattice parameter of 0.536 nm. The thin films deposited on glass have strong absorption at 400 nm.