Photoluminescence (PL) and Raman spectroscopy measurements on GaAs/Si heterostructures grown by molecular beam epitaxy were performed in order to characterize the optical properties of the samples after thermal treatment. The crystalline quality of the annealed GaAs/Si was remarkably improved in comparison with that of the as-grown GaAs/Si. The results of PL spectra for rapidly thermally annealed GaAs/Si showed that the peaks at 1.501 and 1.485 eV, which are related to intrinsic bands, and the peak at 1.467 eV, which can be attributed to impurities, were well resolved. The magnitude of the deformation in the GaAs layer, as determined from the PL measurements, was approximately 2.1 x 10(-3). The ratio of the peak intensity of the longitudinal optical phonon to that of the transverse optical phonon for rapidly thermally treated GaAs/Si increased dramatically in comparison with the corresponding rates for the as-grown GaAs/Si. These results indicate that the crystallinity of the GaAs epilayer is improved by thermal treatment.