Bulk glasses and thin films of Ge20Se50Te30 were prepared by melt-quenching and thermal evaporation technique, respectively. The stoichiometry of the composition was checked by energy dispersive X-ray diffraction (EDX), whereas the crystallization was investigated using differential scanning calorimetery (DSC). The effect of heat treatment on the structure transformation of Ge20Se50Te30 films was determined by X-ray diffraction (XRD). The XRD results reveal that the as-prepared films are amorphous in nature while the annealed ones show crystalline phases. Further, the average crystallite size, strain, and dislocation density were found to depend on the annealing temperature. The optical transmittance and reflectance of the studied films at different annealing temperatures were measured using spectrophotometer. The optical parameters were calculated as a function of annealing temperature. The optical transition was found to be allowed indirect transition with optical band gap decreases from 1.69 to 1.41 eV with increasing the annealing temperature from 553 to 633 K.