The post-growth thermal annealing effects of Mg doped GaAs epitaxial layers on the microstructural and optical properties grown by molecular beam epitaxy (MBE) have been investigated. The properties of Mg doped GaAs are estimated after the process of rapid thermal annealing (RTA) in the temperature range of 600 approximately 750 degrees C. The photoluminescence (PL) peak position of as-grown sample blueshifted from 1.473 to 1.485 eV as well as the pronounced enhancement in PL intensity by annealing at 600 degrees C. In the sample grown at the temperature of T(s) = 475 degrees C, the full-width at half maximum (FWHM) of double crystal X-ray diffraction (DCXRD) decreased form 27 to 8 arcsec with increasing of annealing temperature (600 approximately 700 degrees C). The crystalline quality variation of Mg doped GaAs layers by RTA is greatly dependent upon the doping level.