Amorphous carbon nitride (a-C:N) thin films have been prepared on silicon as low-dielectric-constant materials using an electron cyclotron resonance plasma with an application of a negative r.f. bias to the silicon substrate in a mixture of C2H2 and N, as precursors. The dielectric constants (k) of a-C:N thin films can be as low as 2.0 at 1 MHz in this work. The thermal stability of the film has been improved by the incorporation of nitrogen atoms into the amorphous carbon network with addition of Ar into precursors. The basic structure, composition and electronic properties of these films were analyzed by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), field emission scanning electron microscopy (FE-SEM), and dielectric constant measurements. The dielectric constant of a-C:N thin films can be significantly lowered by the formation of the cross-linked structure and tetrahedral C-N bonds in the film due to the increase of the N/C ratios, and sp(3)-bonded carbon contents in the a-C:N thin films with the increase of precursor pressure from 5 to 8 mtorr.