Optical spectrum measurements are commonly used for the routine determination of thin film optical constants. This paper presents some new methods of evaluating transmission spectrum data only, leading to thickness and values for the complex refractive index by optimizing the physical thickness, refractive index and extinction coefficient, such as step constrained optimization approach, subsection fitting, accelerating optimization model (namely, some dispersion formulae are adopted to comply with the physical constrains set), etc. The methods applies to all kinds of transmission spectra and do not rely on the existence of interference fringe patterns or transparency. Through a lot of experiments of the films Ta<sub>2</sub>O<sub>5</sub>, TiO<sup>2</sup> under different IAD (ion-assisted deposition) conditions, the deviation between the measured and the theoretical spectra data is less than 0.53% in p-order model merit function (<i>p</i>=2), which shows that these methods are reliable. Now, we have gotten the thickness and complex refractive index of DLC (Diamond-Like Carbon) films on BAK7 glass substrates by these methods successfully. Examples are presented and discussed.