The optical properties of monolayer Ge2Sb2Te5 thin films with three different thicknesses prepared by dc magnetron sputtering method at the range of 400-800 nm were studied. The optical absorption coefficients and the optical energy gap (E_(g)) were calculated. The results gave values for the absorption coefficients in the range of (1.3-7.5)*10^(5) cm^(-1) which were in the high absorption wavelength region of 400-800 nm. The optical energy gaps were 0.684, 0.753 and 0.810 eV corresponding the films thicknesses of 57, 88 and 127 nm, respectively, showing the characteristic of increasing with the increase of the film thickness.