Spectroscopic ellipsometry (SE) and spectrophotometer are used to measure the ellipsometric parameters (y and D ) and transmittance of ultrathin diamond like carbon (DLC) film and amorphous silicon (a-Si) film respectively. Accurate determination of optical constants and thickness of DLC film and a- Si film is a difficult problem when ellipsometric parameters are used independently. It is due to the strong statistical correlation which exists between the optical constants and thickness of the film when these parameters are adjusted to fit the experimental ellipsometric parameters. By simultaneous fitting ellipsometric parameters and transmittance (SE+T), thickness and optical constants can be obtained easily and rapidly. However, noise, minor contamination on the film surface or any small absorption in the substrate will be mixed into the film′s optical constants. In order to determine the optical constants of ultrathin DLC and a-Si film accurately, the optical constants are parameterized by combining SE+T and optical constant parameterization approach. The results show that the adopted method is able to uniquely and accurately measure optical constants from a unique solution while maintaining smooth, continuous, and often Kramers-Kronig (K-K) consistent optical properties for the ultrathin DLC film and a-Si film. This method is expected to be of value for optical property measurement of other amorphous films with thickness of just a few tens of nanometers.