Objective To explore accurate measurement of optical constants of the ultrathin tetrahedral amorphous carbon (ta-C) films by combined usage of spectral ellipsometry (SE) and spectrophotometry. Methods As the amount of unknown parameters determined by ellipsometry method was larger than the number of equations and the ellipsometric equations had no unique solution, the accurate determination of the optical constants of the ultrathin ta-C film accurately was difficult when independently using ellipsometric parameters for fitting due to the strong statistical influence of the film thickness on refractive index and extinction coefficient. Therefore, in this paper, ellipsometric parameters and transmittance (hereinafter referred to as SE+T) were simultaneously fitted to obtain optical constants more easily and rapidly. Results The results showed that the film had typical characteristics of amorphous carbon films. There were obvious differences in optical constants obtained by SE and SE+T. The maximum difference value of extinction coefficient k was up to 0. 020 in the visible and infrared area and was 0. 005 in the ultraviolet area. The maximum difference value of the refractive index n was 0. 04 at wavelengths above 500 nm. The n value tended to be consistent by the two methods in the ultraviolet and visible area. The fitting results had better uniqueness by SE+T and the fitted optical constants were smooth. Conclusion This method of SE+T is suitable for accurate measurement of optical constants of ultrathin ta-C films within measurement range.