Photoluminescence measurements at 4.2 °K were used to study the emission behavior and the annealing characteristics in Mg‐ion‐implanted layers in GaAs. Radiative recombination due to the donor‐acceptor pairs and free electrons with holes bound to acceptors involving Mg was observed. The ionization of Mg is esti mated to be 28±2 meV. The donor‐acceptor pair band shows a large energy shift with the change of the excitation intensity. Annealing at the temperatures 750–900 °C sufficient to optically activate Mg ions implanted and to recover from lattice damages.