The optical property of the tetrahedral amorphous carbon (ta-C) films deposited by the filtered cathodic vacuum are (FCVA) technique has been investigated using a spectroscopic ellipsometer. The structure of these films deposited on silicon wafer was simulated by a four-layer model consisting of a roughness layer, a ta-C layer a graded ta-C:Si layer and the silicon substrate. The optical properly of the ta-C layer was derived based on Forouhi and Bloomer amorphous semiconductor model. The graded layer consisting of the mixture of ta-C anti silicon simulates the carbon ion impinging/diffusion into the surface of the silicon substrate. The dielectric constants of the random mixture microstructure were modeled by the effective medium approximation. From the fitting, the F.B. parameters for the ta-C film have been obtained following their physical significance. The complex refraction index, N = n - ik over the range of 1.5-4.2 eV, for ta-C has been determined. The Tauc (optical) gap of 2.5-2.7 eV is obtained from ta-C with 79-88% sp(3) fraction in the film. Similarity in trend is observed from the Tauc gap, F.B., optical band gap, parameter B and sp(3) fraction plotted against the car bon ion energy. The change of the slope of the optical absorption edge is observed when the ion energy is varied.