Abstract Amorphous Bi5Ge40Se55 thin films have been deposited by thermal evaporation technique onto chemically cleaned glass substrates. The influence of thermal annealing on the optical properties of amorphous Bi5Ge40Se55 films has been investigated. X-ray diffraction analysis (XRD) revealed that the as prepared film has an amorphous nature while the annealed films shows two crystalline phases GeSe2 and Bi2Se3 in monoclinic and hexagonal crystal structure with a preferential orientation along 〈125〉 direction. Optical reflectance and transmittance of the as prepared and annealed films were measured in the wavelength range 300–900nm using a double beam spectrophotometer. The optical band gap Eg decreased with increasing of the annealing temperature. This behavior of Eg was discussed in terms of the width of localized stat i.e. Mott and Davis model for amorphous–crystalline structure transformation. Some of the important optical constants such as optical dispersion energies; Eo and Ed, dielectric constants; ε∞ and εL as well as the ratio between number of charge carriers and effective mass n/m* have been evaluated.