Thermoelectric power of annealed β-Ag2Se thin films of different thicknesses has been measured both while heating and cooling by the integral method. It is found that it remains practically constant (in β-Ag2Se phase) during heating while it is a function of temperature while cooling. The thermoelectric power in both heating and cooling cycles is a function of inverse thickness of the films. The difference in behavior between Ag2Se films during heating and cooling is attributed to the possible transformation to monoclinic phase during cooling from the original orthorhombic phase during heating. The inverse thickness dependence has been explained by the size effect theories. Important material parameters like carrier concentration, Fermi energy, effective mass of carriers, and energy dependence of the mean free path have been evaluated for the β-Ag 2Se (orthorhombic) phase.