Thin films of Ge30Se50Te20 with thickness of 100, 150, 200 and 250 nm were prepared by thermal evaporation technique on glass substrates. Normalincidence optical transmission spectra have been measured in the range from 190 to 900 nm. The optical constants were calculated using Swanepoel's method. The effect of film thickness on the optical constants had been investigated. Analysis of the refractive index (n) yields the values of the high frequency dielectric constant (∞), Wemple's single oscillator energy Ew, dispersion energy Ed, the oscillator wavelength (o), the average oscillator strength (So) and carrier concentration N/m* at different thickness. Optical absorption measurements showed that fundamental absorption edge is a function of the film thickness. The optical gap energy Eg is determined. The variation of the optical constants with film thickness was reported. The results are interpreted on the basis of the local field correction theory