Amorphous ternary $Si_xC_yN-z$ thin films were obtained by plasma enhanced chemical vapor deposition(PECVD) using $N_2, SiH_4 ;and ;C_2H_4$ as the reaction sources. The chemical state were characterized by x-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy(FTIR). The optical properties of the thin films were investigated by UV-visible spectrophotometer and ellipsometer, and the optical band gaps of thin films were determined from corresponding transmittance spectra following Tauc equation.