Amorphous carbon nitride films have been synthesized on silicon using an electron-cyclotron-resonance chemical vapor deposition (ECR-CVD) system combined with a negative d.c. bias in a mixture Of C2H2,N-2 and Ar as precursors. The refractive index and extinction coefficient of the amorphous carbon nitride films were determined by the N&K analyzer. The optical band gap was derived according to Tauc's equation. The optical constants (n, k) and structural properties of the amorphous carbon nitride films were studied as a function of the substrate negative d.c. bias, ECR-power, flow rate ratio (N-2/C2H2), nitrogen to carbon ratio (N/C) by X-ray photoelectron spectroscopy (XPS), degree of graphitization (sp(2) content) by Raman spectroscopy and the radicals' density in the ECR-plasma by the optical emission spectrometer (OES). Through the relationship between OES measurements and the Raman studies with optical band gap, the result indicated that a progressive graphitization of the film occurs with increasing N2/C2H2, N/C, ECR-power and substrate negative d.c. bias. The optical constants (n, k) of the a-C:N films prove an unambiguous dependence of the optical band gap on the substrate negative d.c. bias for the ECR-power.