Thin films of Se87.5Te10Sn2.5 were prepared by vacuum thermal evaporation technique. Various optical constants were calculated for the studied composition. The mechanism of the optical absorption follows the rule of direct transition. It was found that the optical energy gap (Eg) decreases from 2.26 to 1.79 eV with increasing the annealing temperature from 340 to 450 K. This result can be interpreted by the Davis and Mott model. On the other hand, the maximum value of the refractive index (n) is shifted towards the long wavelength by increasing the annealing temperature. In addition, the high frequency dielectric constant (εL) increased from 31.26 to 48.11 whereas the ratio of the free carriers concentration to its effective of mass N/m⁎ decreased from 4.3 to 2.09 (×1057 (m−3 Kg−1)). The influence of annealed temperature on the structure was studied by using the X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD studies show that the as-deposited films are amorphous in nature, but the crystallinity improved with increasing the annealing temperature. Furthermore the particle size and crystallinity increased whereas the dislocation and strains decreased with increasing the annealing temperature. SEM examination showed that the annealing temperature induced changes in the morphology of the as-deposited film.