Reflection high-energy electron diffraction (RHEED), double-crystal X-ray rocking curve (DCRC), and photoluminescence (PL) measurements were performed to investigate the effect of thermal annealing on the structural and the optical properties of CdTe (111) epilayers grown on GaAs (100) substrates by molecular beam epitaxy (MBE) at low temperature. The results of the RHEED patterns showed that the oxidized layer on the GaAs substrate was removed in a Te atmosphere, and that the 20-Angstrom CdTe layer was grown by three-dimensional process. When the rapid thermal annealing (RTA) was performed at 500 degrees C for 14 s, the FWHM of the DCRC for the CdTe layer had the smallest value. After the RTA process, the luminescence intensity of the exciton remarkably increased, and the peak at 1.476 eV was dominant. As the RTA temperature increased, the luminescence intensity of the exciton peak related to neutral acceptors (A degrees, X) increased. The temperature dependence of the spectra showed that the (A degrees, X) peak originated from the recombination of the excitons bound in high-density defects. The excitation power intensity dependence of the PL spectra should that the peaks of the transitions due to donor-acceptor pairs shifted to larger energies. These results indicate that the structural and the optical properties of the CdTe epilayers grown on GaAs (100) are improved by RTA, and that the RTA process is very useful for the growth of HgxCd1-xTe on CdTe/GaAs heterostructures.